So, since I critiqued the OP's interpretation of a cosmology, here's mine. Have at it if you want, or adopt it if it works.
My starting assumption was to use the cosmology as explained in the 5e PHB appendix C, the great wheel. I went with the great wheel because it was the only one that made sense. The tree just seems stupid, and the world axis has the Abyss, and therefore demons, in a place completely unrelated to literally every other part of the outer planes. The description literally says divine domains are in the astral sea, up top, and then states in brackets that what they mean by divine domains in this sense are the Outer Planes, which would normally include the Abyss. Except the Abyss is now a hole in the bottom of both the Elemental Chaos and the fabric of reality. Not sure how you tear a hole in that, less sure how creatures could exist in said hole, but most importantly, this makes demons innately different from Devils, Celestials, Modrons, Slaadi, whatever. They live in a hole in the elemental chaos, everything else is on the outer planes. Even the Slaadi, who are fundamentally chaotic, are up there, but not the demons. So... why is there a Blood War? If the demons invade anything on the world axis it would be the material echoes, not the outer realms. Where's the conflict? Why don't the Celestials just stomp the Devils, now that there's no reason for Primus to rule that they
(Asmodeus in particular) are a necessary evil to keep the demons at bay? The world axis is clearly modeled on heaven and hell, but they tried to work the astral sea and the elemental planes into the mix and ended up with a mess. Still better than the tree though. So, Great Wheel it is.
Now, I built this cosmology in conjunction with trying to redesign the 5e classes in general. I got about half way through and was fairly happy with my results so far, then I turned my attention to the Cleric. And that meant considering the Paladin and Warlock, as well, and naturally, I tried to make sense of the disparate ways of getting power from an outside source by means of established contracts-in one form or another, whether that be an Oath, a Pact, or a Communion with a deity, and that led to trying to build a consistent cosmological model to put them all in, and there we go.
I started with the simplest model. Positive Plane on the top, Negative Plane on the bottom, Material Plane in the middle. Given what the established lore of the Positive and Pegative Planes is, it was incredibly easy to slot the Material Echoes in on either side of the Material Plane. The Feywild is closer to the Positive Energy Plane, and the Shadowfell is closer to the Negative Energy Plane. That intuitively explains why they exist, no need for the gods to sort all the stuff that was 'too bright' or 'too dark' out of the material plane.
In fact, no need for creator deities at all. That's a specific feature. I don't want to attach any gods to the making of the cosmology-every single cosmic entity that could interact with the game setting is a part of the universe, not an outside creator (though, if a character were on good terms with a god, and asked them if gods prayed to anything, they might be told that some of the gods view the Positive or Negative planes (or both) in a vaguely religious light, or else believe in a sort of creator god called 'De'hm', or 'Ge'hm', though none have met them personally, and some believe other things about their origins). My reason for that is simple. I don't know which god, gods, or forces are responsible for the universe existing in a campaign, if any, so it has to be a general sort of explanation that can work with anything.
Anyway, this was already enough cosmology for my current purpose, which was trying to understand the Cleric class and its relation to divinity. Something that particularly irked me was the fact that all Clerics, whether their domain was Light, or Trickery, or War, or Arcana, or Truth or Chaos (those last two are mine, don't bother googling them), were equipped with Turn Undead and its improvements. 'What, does every god, regardless of their domain, regardless of whether they are Good or Lawful or Chaotic, specifically have a problem with Undead?' Then I had a moment of epiphany. Yes. Yes they do. That is precisely the point of the Cleric Class. When you sign up to be a Cleric, you are involving yourself in a war more fundamental to the nature of the universe than the Blood War. You volunteer to be both the eyes and ears of the deities to warn against, and first responder to fight against, incursions of the undead. You, the cleric, might die in the process, but you might delay or halt the incursion that took you down, and at the very least give the gods warning so they can send a more powerful person or group to deal with the problem, or at least evacuate/shore up key areas. That's what Clerics are for. This explains why all clerics get Turn Undead very early, and Divine Intervention-the only class feature that specifically, unambiguously, supports the idea that you have a literal personal relationship with a deity, the very flavour of the cleric class, only at level ten.
The level progression now looks something like this: at level one, you are a part of a religious tradition that has trained you to draw on divine power, presumably in the name of a particular god, but there isn't really any firm guarantee said god is actively involved with you gaining said divine power-it could be a different god, or the power of organised religious belief or personal devotion, or an impersonal numinous 'divine', or even a Fiend pretending to be a god. It's clear that something is empowering you, but it's a matter of faith as to exactly who or what that is.
Then, at level two, as a properly seasoned holy warrior, you gain the ability to channel divine power in a more direct and immediate way than casting spells. You can use it to do one of two things. One is to Turn Undead, the other is a seemingly random ability that aligns with whatever domain you most identify with. All Clerics turn undead, but only some Clerics do this or that Channel Divinity feature. In a simple order of priority, Turn Undead is a feature more important to the basic functioning of the class than the other, even though we normally associate more with the domain as our means of identifying a cleric.
A simple in universe explanation for that which I find holds up quite well under scrutiny is that the human and divine understandings of the Cleric class differ. From the divine perspective, a Cleric is a Cleric. Turn Undead is their duty, regardless of domain. The other channel divinity, and in fact all Domain features, is not a class defining choice. It's adequate compensation. Core features are the duty, Domain features are compensation, whether the Cleric sees it that way or not. Why are there different domains? Different gods have different ways of appealing to people, based on what they personally prefer and what experience tells them is likely to work best. If there were a single, universal compensation for services rendered, there would be no subclasses of Cleric, because it wouldn't be necessary.
This immediately presents a systemic viewpoint. The gods, or at least enough of them, decided that the undead were enough of a problem to do something about it, and so one or more proposed a joint venture-an interalignnent joint venture, even. They would front up the power to sustain a pool of shared divine energy with which to empowerer this new class of Cleric that they decided needed to exist. Over debate, the rules were hashed out, the method was theorised and possibly tested, and then they got final approval and support from enough deities from across the alignment spectrum-maybe even some evil ones, acting in enlightened self interest or just for grins and giggles. I have ideas for the details but I won't attempt to outline them here. So then, the Cleric initiative was formed, and empowered, and permanently set in place in such a way that each deity was burdened with its upkeep in direct proportion to how many of the proposed and accepted domains they decided to sponsor. And then they introduced the idea to mortals that there was divine power to be connected with, and people started doing it, and teaching the same, and over the natural course of years religious organisations formed and this of course meant more clerics, and more perceived profitability to the deities so more were willing to invest in the project, which meant more power available to sustain more active clerics, and so on.
So, as a Cleric levels up, not only do they gain more natural facility with using their divinely given powers, as any class would, but they also gain an increasing level of divine favour, such that at level ten they have access to Divine Intervention. So, at some point, and here we switch from mechanics to story, there is a progression from 'a cleric, one of many', to 'my/our cleric, whom I/we have chosen'. So what makes the difference? In Level Up terms, Prestige. One god may be genuinely associated with a given Cleric's earthly religion, and naturally feel an increased affinity for said Cleric as they advance in levels and prove their devotion, which they reward. Other gods may attempt to win the devotion of a cleric with a domain that they support, or even one from a different domain that had proven worth, in a manner similar to a divine job offer (or competing job offers, now there's an idea).
Anyway, this whole system needs a motive. Why is fighting the undead so important? The answer comes naturally from the cosmology I've already established. The Positive and Negative Planes in opposition, the material echoes in between. It becomes clear that the various deities are, regardless of their alignment, in a general sense powered by and dependent on the Positive Energy Plane. So, if the Feywild is the Material Plane made more aligned with the Positive Energy Plane, doesn't it stand to reason that the gods, who are strongly aligned with that same plane, are to the Feywild what the Feywild is to the Material Plane? And so, I place the Outer Planes above the Feywild this cosmology, making no issue of alignment at this time as it seems to me to be a matter of philosophical leanings within the context of the Outer Planes, and so it doesn't seem appropriate to suggest that Lawful Good for example is more 'up' than Chaotic Good or True Neutral or Neutral Evil, relative to the Feywild and Material Plane. And so, with all the major players, Celestials, Demons, Devils, Whatever, clustered at one end of the spectrum, and at least some of their interest specifically focused on the undead, at the opposite end of the spectrum, which they consider an important enough issue to create a Class for mortals that is defined by access to divine power and a set of abilities designed for dealing with the undead specifically, the question to ask is 'why?'.
The answer is already found in established 5e lore. Straud. Gulthias. Who created them? Gulthias I am just assuming, but Straud is specifically mentioned to have be given his powers by and also imprisoned within and with Barovia by, entities known as the 'Dark Powers'. Now we know Straud bodied Mordenkainen, so they are strong enough to grant powers strong enough to do that, and also strong enough to create a prison demiplane to hold a being with such powers. And said demiplane is properly a domain of dread, which in 5e is explicitly tied to the Shadowfell. Whether or not your setting has Straud or someone like him, in a general sense these Dark Powers would logically be associated with the Shadowfell, and in fact would have the same relationship with it as the deities have with the Feywild. In which case, we have what basically amounts to a rival pantheon, or a rival alignment, perhaps, more closely aligned with the negative energy plane, and particularly associated with the creation of new, powerful undead. And by extension, probably the creation of undead in general. And so, despite their differences, all the positively-aligned groups see the incursions of undead as a threat to their interests on the material planes, at the very least, due to the force behind them, and so Clerics were proposed as a solution to that problem. Perhaps one of many, perhaps the sole line of defense, who knows. Meanwhile, the Blood War and the Lawful-Chaotic assaults on each other and whatever else is going on in the Outer Planes continues more or less uninterupted. We now have an unambiguously evil (life-denying) group in the Dark Powers, and that allows us to have more nuance and moral complexity in the dealings of the Outer Planes (life-affirming, in a general sense, but otherwise divided into factions along moral and philosophical grounds that mortals struggle to apprehend the totality of, and can never fully understand).
And now, given all this, I'll add one more detail to the cosmology. The Outlands is obviously in the same general metaphysical space as the Outer Planes, since it is the gateway to them all. As such, it belongs above the Feywild, closer to the Positive Energy Plane. And, since the 5e DMG explicitly states that "the Outlands is the closest the Outer Planes come to being like a world on the Material Plane", why don't we make that a statement of true cosmological significance and formally define the Outlands as a Material Echo, in the same sense as the Feywild and the Shadowfell. At this point, it is likely appropriate to call these four planes 'the Material Planes', and not worry so much about the terminology of 'echoes', which
My starting assumption was to use the cosmology as explained in the 5e PHB appendix C, the great wheel. I went with the great wheel because it was the only one that made sense. The tree just seems stupid, and the world axis has the Abyss, and therefore demons, in a place completely unrelated to literally every other part of the outer planes. The description literally says divine domains are in the astral sea, up top, and then states in brackets that what they mean by divine domains in this sense are the Outer Planes, which would normally include the Abyss. Except the Abyss is now a hole in the bottom of both the Elemental Chaos and the fabric of reality. Not sure how you tear a hole in that, less sure how creatures could exist in said hole, but most importantly, this makes demons innately different from Devils, Celestials, Modrons, Slaadi, whatever. They live in a hole in the elemental chaos, everything else is on the outer planes. Even the Slaadi, who are fundamentally chaotic, are up there, but not the demons. So... why is there a Blood War? If the demons invade anything on the world axis it would be the material echoes, not the outer realms. Where's the conflict? Why don't the Celestials just stomp the Devils, now that there's no reason for Primus to rule that they
(Asmodeus in particular) are a necessary evil to keep the demons at bay? The world axis is clearly modeled on heaven and hell, but they tried to work the astral sea and the elemental planes into the mix and ended up with a mess. Still better than the tree though. So, Great Wheel it is.
Now, I built this cosmology in conjunction with trying to redesign the 5e classes in general. I got about half way through and was fairly happy with my results so far, then I turned my attention to the Cleric. And that meant considering the Paladin and Warlock, as well, and naturally, I tried to make sense of the disparate ways of getting power from an outside source by means of established contracts-in one form or another, whether that be an Oath, a Pact, or a Communion with a deity, and that led to trying to build a consistent cosmological model to put them all in, and there we go.
I started with the simplest model. Positive Plane on the top, Negative Plane on the bottom, Material Plane in the middle. Given what the established lore of the Positive and Pegative Planes is, it was incredibly easy to slot the Material Echoes in on either side of the Material Plane. The Feywild is closer to the Positive Energy Plane, and the Shadowfell is closer to the Negative Energy Plane. That intuitively explains why they exist, no need for the gods to sort all the stuff that was 'too bright' or 'too dark' out of the material plane.
In fact, no need for creator deities at all. That's a specific feature. I don't want to attach any gods to the making of the cosmology-every single cosmic entity that could interact with the game setting is a part of the universe, not an outside creator (though, if a character were on good terms with a god, and asked them if gods prayed to anything, they might be told that some of the gods view the Positive or Negative planes (or both) in a vaguely religious light, or else believe in a sort of creator god called 'De'hm', or 'Ge'hm', though none have met them personally, and some believe other things about their origins). My reason for that is simple. I don't know which god, gods, or forces are responsible for the universe existing in a campaign, if any, so it has to be a general sort of explanation that can work with anything.
Anyway, this was already enough cosmology for my current purpose, which was trying to understand the Cleric class and its relation to divinity. Something that particularly irked me was the fact that all Clerics, whether their domain was Light, or Trickery, or War, or Arcana, or Truth or Chaos (those last two are mine, don't bother googling them), were equipped with Turn Undead and its improvements. 'What, does every god, regardless of their domain, regardless of whether they are Good or Lawful or Chaotic, specifically have a problem with Undead?' Then I had a moment of epiphany. Yes. Yes they do. That is precisely the point of the Cleric Class. When you sign up to be a Cleric, you are involving yourself in a war more fundamental to the nature of the universe than the Blood War. You volunteer to be both the eyes and ears of the deities to warn against, and first responder to fight against, incursions of the undead. You, the cleric, might die in the process, but you might delay or halt the incursion that took you down, and at the very least give the gods warning so they can send a more powerful person or group to deal with the problem, or at least evacuate/shore up key areas. That's what Clerics are for. This explains why all clerics get Turn Undead very early, and Divine Intervention-the only class feature that specifically, unambiguously, supports the idea that you have a literal personal relationship with a deity, the very flavour of the cleric class, only at level ten.
The level progression now looks something like this: at level one, you are a part of a religious tradition that has trained you to draw on divine power, presumably in the name of a particular god, but there isn't really any firm guarantee said god is actively involved with you gaining said divine power-it could be a different god, or the power of organised religious belief or personal devotion, or an impersonal numinous 'divine', or even a Fiend pretending to be a god. It's clear that something is empowering you, but it's a matter of faith as to exactly who or what that is.
Then, at level two, as a properly seasoned holy warrior, you gain the ability to channel divine power in a more direct and immediate way than casting spells. You can use it to do one of two things. One is to Turn Undead, the other is a seemingly random ability that aligns with whatever domain you most identify with. All Clerics turn undead, but only some Clerics do this or that Channel Divinity feature. In a simple order of priority, Turn Undead is a feature more important to the basic functioning of the class than the other, even though we normally associate more with the domain as our means of identifying a cleric.
A simple in universe explanation for that which I find holds up quite well under scrutiny is that the human and divine understandings of the Cleric class differ. From the divine perspective, a Cleric is a Cleric. Turn Undead is their duty, regardless of domain. The other channel divinity, and in fact all Domain features, is not a class defining choice. It's adequate compensation. Core features are the duty, Domain features are compensation, whether the Cleric sees it that way or not. Why are there different domains? Different gods have different ways of appealing to people, based on what they personally prefer and what experience tells them is likely to work best. If there were a single, universal compensation for services rendered, there would be no subclasses of Cleric, because it wouldn't be necessary.
This immediately presents a systemic viewpoint. The gods, or at least enough of them, decided that the undead were enough of a problem to do something about it, and so one or more proposed a joint venture-an interalignnent joint venture, even. They would front up the power to sustain a pool of shared divine energy with which to empowerer this new class of Cleric that they decided needed to exist. Over debate, the rules were hashed out, the method was theorised and possibly tested, and then they got final approval and support from enough deities from across the alignment spectrum-maybe even some evil ones, acting in enlightened self interest or just for grins and giggles. I have ideas for the details but I won't attempt to outline them here. So then, the Cleric initiative was formed, and empowered, and permanently set in place in such a way that each deity was burdened with its upkeep in direct proportion to how many of the proposed and accepted domains they decided to sponsor. And then they introduced the idea to mortals that there was divine power to be connected with, and people started doing it, and teaching the same, and over the natural course of years religious organisations formed and this of course meant more clerics, and more perceived profitability to the deities so more were willing to invest in the project, which meant more power available to sustain more active clerics, and so on.
So, as a Cleric levels up, not only do they gain more natural facility with using their divinely given powers, as any class would, but they also gain an increasing level of divine favour, such that at level ten they have access to Divine Intervention. So, at some point, and here we switch from mechanics to story, there is a progression from 'a cleric, one of many', to 'my/our cleric, whom I/we have chosen'. So what makes the difference? In Level Up terms, Prestige. One god may be genuinely associated with a given Cleric's earthly religion, and naturally feel an increased affinity for said Cleric as they advance in levels and prove their devotion, which they reward. Other gods may attempt to win the devotion of a cleric with a domain that they support, or even one from a different domain that had proven worth, in a manner similar to a divine job offer (or competing job offers, now there's an idea).
Anyway, this whole system needs a motive. Why is fighting the undead so important? The answer comes naturally from the cosmology I've already established. The Positive and Negative Planes in opposition, the material echoes in between. It becomes clear that the various deities are, regardless of their alignment, in a general sense powered by and dependent on the Positive Energy Plane. So, if the Feywild is the Material Plane made more aligned with the Positive Energy Plane, doesn't it stand to reason that the gods, who are strongly aligned with that same plane, are to the Feywild what the Feywild is to the Material Plane? And so, I place the Outer Planes above the Feywild this cosmology, making no issue of alignment at this time as it seems to me to be a matter of philosophical leanings within the context of the Outer Planes, and so it doesn't seem appropriate to suggest that Lawful Good for example is more 'up' than Chaotic Good or True Neutral or Neutral Evil, relative to the Feywild and Material Plane. And so, with all the major players, Celestials, Demons, Devils, Whatever, clustered at one end of the spectrum, and at least some of their interest specifically focused on the undead, at the opposite end of the spectrum, which they consider an important enough issue to create a Class for mortals that is defined by access to divine power and a set of abilities designed for dealing with the undead specifically, the question to ask is 'why?'.
The answer is already found in established 5e lore. Straud. Gulthias. Who created them? Gulthias I am just assuming, but Straud is specifically mentioned to have be given his powers by and also imprisoned within and with Barovia by, entities known as the 'Dark Powers'. Now we know Straud bodied Mordenkainen, so they are strong enough to grant powers strong enough to do that, and also strong enough to create a prison demiplane to hold a being with such powers. And said demiplane is properly a domain of dread, which in 5e is explicitly tied to the Shadowfell. Whether or not your setting has Straud or someone like him, in a general sense these Dark Powers would logically be associated with the Shadowfell, and in fact would have the same relationship with it as the deities have with the Feywild. In which case, we have what basically amounts to a rival pantheon, or a rival alignment, perhaps, more closely aligned with the negative energy plane, and particularly associated with the creation of new, powerful undead. And by extension, probably the creation of undead in general. And so, despite their differences, all the positively-aligned groups see the incursions of undead as a threat to their interests on the material planes, at the very least, due to the force behind them, and so Clerics were proposed as a solution to that problem. Perhaps one of many, perhaps the sole line of defense, who knows. Meanwhile, the Blood War and the Lawful-Chaotic assaults on each other and whatever else is going on in the Outer Planes continues more or less uninterupted. We now have an unambiguously evil (life-denying) group in the Dark Powers, and that allows us to have more nuance and moral complexity in the dealings of the Outer Planes (life-affirming, in a general sense, but otherwise divided into factions along moral and philosophical grounds that mortals struggle to apprehend the totality of, and can never fully understand).
And now, given all this, I'll add one more detail to the cosmology. The Outlands is obviously in the same general metaphysical space as the Outer Planes, since it is the gateway to them all. As such, it belongs above the Feywild, closer to the Positive Energy Plane. And, since the 5e DMG explicitly states that "the Outlands is the closest the Outer Planes come to being like a world on the Material Plane", why don't we make that a statement of true cosmological significance and formally define the Outlands as a Material Echo, in the same sense as the Feywild and the Shadowfell. At this point, it is likely appropriate to call these four planes 'the Material Planes', and not worry so much about the terminology of 'echoes', which
