So, here's the basics of the Deep Ethereal Plane and traveling. Some of this has already been brought up.
1) Traditionally, the Deep Ethereal Plane connects the Material Plane to the Inner Planes and demiplanes. It is not connected to the Outer Planes. The Astral Plane is what connects the Material Plane to the Outer Planes.
2) You eat/sleep normally in the Ethereal Plane. Again, it's the Astral Plane where that's different.
3) Getting to a destination in the Inner Planes and getting to a destination in the Astral Plane are
sort of similar, in that you have a destination in mind and start traveling until you get there.
4) While travel in the Astral Plane is clearly purely directed by intention, the Ethereal version is murkier. Ethereal maps apparently exist, and I get the impression from some materials that perhaps there are certain discernible qualities to the multi-colored strands of ether everywhere that you can follow to your destination (or something like that). Like, for example, maybe if you want to reach the Elemental Plane of Fire, you go the directions where there seem to be more red, orange, and yellow colors, and/or it feels a bit warmer, etc. Personally, I'd include a bit of something like that for flavor, even though the mechanics are similar (roll on a random table to see how long it takes to reach your destination). Maps probably shorten the time.
5) The Border Ethereal is just the little overlap with the Material Plane. You don't travel anywhere through it except to other parts of the Material Plane. So if you want to go to another plane, yes, you do need to go through the Ethereal Plane (though, as was mentioned, that won't get you to the Nine Hells--you'd need the Astral Plane for that).
6) While 3e wanted to make ethereal curtains (also known as walls of color) similar to rectangular astral color pools, that isn't what they have traditionally been. They are simply the membrane around the Border Ethereals of the various planes. So the Elemental Plane of Air has
one ethereal curtain and it's going to look like an infinite wall in front of you when you find it. There are tricks you can do to use it to imprecisely travel around your destination plane at high speed, but it's really just the boundary between the Deep and the local Border, and you can freely pass through it in either direction.
Stepping back a bit, there are some serious problems with how the Ethereal and Astral Planes have been presented since the beginning.
They have always been presented as planes that are between other planes, and which are used for travel between those planes. However, the mechanics of the game have never supported doing so. It's frankly ridiculous. Here is how it works:
-To get to the Deep Ethereal Plane, you can use
etherealness to get to the Border Ethereal, and then move into the Deep Ethereal (5e doesn't provide a mechanism for that, but 2e does, and it amounts to just willing yourself to pass from the Border into the Deep, basically taking an action). However, once
etherealness wears off, it is going to yank you from wherever you are in the Deep Ethereal and slam you back into your origin plane (or maybe another plane that is closer). It doesn't last nearly long enough to actually travel to other planes. It's really intended to "interact" with the Material Plane, by invisibly scouting around, moving through walls, etc, rather than as a travel enhancer.
-Alternatively, you could use
plane shift to get to either the Border or Deep Ethereal, and from there travel through the Deep Ethereal to your destination plane. At your destination plane, you can pass through the wall of color into its Border Ethereal. But there is no built-in means of getting from the Border Ethereal to the overlapping plane, so you would have to cast
plane shift a second time to get there.
This means the entire trip was a complete waste of time since you could have just cast plane shift to go directly to your destination plane without ever traveling through the Ethereal Plane. This
fundamental flaw of the mechanics has been there since the beginning.
-Traveling through the Astral Plane is typically equally as useless as traveling through the Ethereal Plane. You can, again, use
plane shift to get there, and find an astral color pool to your destination (which in this case, actually will send you to your destination if you hop in, without needing to cast another spell), but you could have just used
plane shift and skipped the astral trip. There is the 9th level
astral projection spell that lets you do it in an astral form, which can make it easier to avoid dying on the Outer Plane you arrive at, if you with go with traditional rules (or the little addition hidden in the 2014 DMG that isn't in the spell description itself), but otherwise it's not terribly useful. Also, when you can cast 9th level spells, you can just skip all that nonsense and use
gate to go exactly where you want to go on any plane.
So there has been a fundamental disconnect between the really awesome lore that you use these strange dimensions to travel between other planes, and there being no mechanics to make that actually worth doing. The spells that would let you do that would let you bypass it, or they are equal or higher level than the spells that would let you bypass it.
It bugs me so much that I made a couple spells to make the lore work:
Ether Conduit
5th-level conjuration
Casting Time: 10 minutes
Range: 10 feet
Components: V, S, M (a quartz worth at least 50 gp, which the spell consumes)
Duration: 10 minutes
Drawing from the limitless possibilities of the Ethereal Plane, you conjure a volatile conduit of ether that snakes from the Deep Ethereal Plane, through the Border Ethereal, and grounds itself to a location on a plane that borders the Ethereal Plane.
You can cast this spell either at the Wall of Color in the Deep Ethereal Plane or on a plane that borders the Ethereal Plane.
If you are in the Deep Ethereal Plane when you cast this spell, an opening appears at a point you can see within range on the Wall of Color. If the Wall of Color is not in range, the spell fails. Another opening appears on the bordering plane.
If you are on a bordering plane when you cast this spell an opening appears at a point you can see within range. Another opening appears in the Deep Ethereal Plane on the Wall of Color.
The opening has a front and a back on each plane where it appears. The front is a a 10-foot diameter circular corridor of slowly swirling multi-colored fog. Any creature that enters it is immediately ejected from the corridor on the other plane, appearing in the nearest unoccupied space. The back in the Deep Ethereal merges with the Wall of Color, and the back on the bordering plane is a 10-foot diameter impassable opaque wall of multi-colored fog.
There is a 70% chance that the conduit opens somewhere in the corresponding vicinity on the other plane. Otherwise it opens 1d10 x 1d20 miles away in a random direction.
This spell has no effect if you cast it while you are on the Border Ethereal or on a plane that doesn't border the Ethereal Plane.
Astral Access
6th-level conjuration
Casting Time: 1 action
Range: Touch
Components: V, S, M (a small platinum mirror worth at least 200 gp)
Duration: Instantaneous
You and up to six willing creatures who link hands in a circle are physically transported to the Astral Plane. You appear within 60 feet of a random astral color pool leading to your plane of origin.
This spell has no effect if you cast it while you are on the Astral Plane or on a plane that doesn't connect to the Astral Plane.
I designed those spells so as to be lower level, but not negate the utility of existing spells. So, for example,
ether conduit is designed to take you from the Material (or Inner) Plane directly to the Deep Ethereal or back again. Since moving between the Border and Deep Ethereal are imprecise, this means you can't use this to jump to exactly where you want to go in the Border Ethereal, and then cast it again to go back to exactly where you want in the Material (which would have given it the functionality of
etherealness at lower level). Instead, this spell is designed precisely to make traveling through the Deep Ethereal to reach other planes be something that actually happens. It is two levels lower than
plane shift, and therefore fulfills a useful niche.
Astral access is one level higher than that, but still an easier way to get to the Astral Plane to start a journey than
plane shift is. It also has the added use of being a quick escape button to get out of a nasty situation, though it doesn't give you an easy way to return to your starting point (the color pool is unlikely to drop you off near your starting location, since it is random).
As one additional point, the 2e Planescape
A Guide to the Ethereal Plane book is the fullest treatment of the Ethereal Plane. It has somewhat confusing language that may seem to suggest that you can go directly from a wall of color in the Deep Ethereal to the actual plane on the other side, rather than the Border Ethereal. It is extremely unclear and potentially contradictory in context. However, I might suggest making there be "shallow" areas on the wall of color that do exactly that. So normally the wall of color takes you back and forth from the Border to the Deep, but there are certain patches of it (which look different) where you can pass from the Deep Ethereal directly to the bordering plane, skipping the Border Ethereal (and not requiring a spell). This makes Ethereal Plane travel even more appealing, since it only requires a single 5th-level spell to get started (which you could hire someone to cast for your party), and you can then reach any destination plane that connects to the Ethereal Plane.
From your mention of the spell schools working differently, you are probably looking at 1e or 2e sources. Even if you are playing one of those editions, I recommend ignoring those particular rules. They are way too fiddly. Instead, just make up one or two interesting ways that spells might work differently on the plane and stick with that.
