I'm amused by how little of what you fell in love with is part of the original Great Wheel. The Feywild and the Shadowfell are pure 4e/World Axis, as for that matter is the Elemental Chaos as opposed to much purer elemental planes. Mechanus is a substitute that doesn't really fit - but is so much stronger than the original Nirvana that no one cares. And Ysgard is just off-brand Asgard, not that there's anything wrong with that.
I know. The only planes from the original Great Wheel that I actually like are Limbo and Mechanus, and I have problems with those, too (like I said earlier, the fact that the embodiments of pure chaos are multicolored-frog people (and not even the only multicolored frog-people in the game; I'm looking at you, Grung) is a big turn-off for me from Limbo. I do like the Githzerai, however). I'm aware that the Feywild and Shadowfell came from 4e (well, kinda. They were demiplanes before 4e). (I also think that Limbo and the Elemental Chaos are too similar, and might as well be combined.)
7 Heavens is so someone can say they are in 7th Heaven. The Nine Hells (not seven) started as the Nine Circles of Hell in Dante's Inferno.
I meant "seven versions of hell and heaven" as there being 7 different upper planes (heaven) and 7 different lower planes (hell). I'm aware of the origin of the Nine Hells. Just from a design standpoint, I don't see the need to have 7 different Lower Planes and 7 different Upper Planes. Hell, one Lower Plane and one Upper Plane would work with the sub-planes that nearly every Great Wheel plane has.
Half of that's in the Feywild or Shadowfell. Why there wasn't before 4e? Pass. There's a reason fluff wise I consider it goes 4e> 5e > everything before.
IMO, the Plane of Dreams is a pretty distinct concept from the Feywild, as is the Plane of Nightmares and Plane of Death from the Shadowfell. Twilight Forest could be a part of the Feywild (like one of the new Domains of Delight), but it could also be its own thing (as Eberron demonstrates by merging the Beastlands with a Twilight Forest).
The difference between a lightly satirical version of Narnia and Faerie.
Yeah, I don't see the need for that. Narnia already has a ton of Fey-creatures (Satyrs/Fauns, Centaurs, Nymphs, Merfolk, etc), so they might as well be one and the same.
Slaad made it to D&D first.
Slaad had their own plane of existence first. Fey have been around since the beginning, and have always been known for their chaotic and tricksy nature.
Different approaches. In Valhalla war is almost fun. Acheron is much grimmer. This is one I can actually see. Of course I once again prefer the 4e World Axis domains of war because one invaded the other.
Then just make one plane of war with cosmic different battlefields

One for Vikings where they fight Valhalla-style, one for Goblinoids vs. Orcs, one for the Blood War, one for Celestials vs. Fiends, etc. Redundancy doesn't help anyone, IMO.
They're finally answering the Plane of Dragons question in Fizbans, saying that the Material Plane is the Plane of Dragons (created by the dragon gods and the dragons are native to it), though it is very late into the hobby to be giving a sort of explanation like this, IMO. Giants? Notsomuch.
Because Olympus isn't supposed to be infinitely big. Mount Olympus is in Arvendor (CG)
Then just make Mount Olympus infinitely big. Deities live on it, for Heavens' sakes! (pun intended)
What makes me even more mad/confused is that there's Hades (which isn't the Underworld, for some reason, and doesn't have Hades/Pluto in it), the Shadowfell, and the Fugue Plane that all sort of function as a "Plane of Death" (at least name/concept-wise), even though Hades is the most irrelevant of the 3.
If we have Neutral Evil Fiends (itself a little box-ticky) they gotta have a domain. But no idea other than they wanted to put the anchors for Mount Olympus and Yggdrasil right at the bottom of the wheel.
Kinda my point. Yugoloths are box-ticky, and Gehanna exists only because of box-ticking (and it's not even in the right spot!).
There is. In 4e it's called the Astral Sea and in 5e it's called the Great Wheel and goes on the outside of the axis.
Well, the Great Wheel isn't really a plane of existence, it's a way of organizing the planes (and the Astral Sea/Elemental Chaos is kinda the same). What I meant was more "why not just have a few Upper Planes and a few Lower Planes (probably not called that anymore and organized differently) and get rid of all the filler?".