I don't recall that specific detail, but, yes, now that you mention it, the Astral did connect the inner and outer planes, as well as the outer, I guess I was over-simplifying..
Definitely don't remember those. Parallel prime material planes always struck me more as an excuse for having many different campaigns out there be theoretically in the same multiverse, without actually needing to interact. :shrug: Guess that's just how I liked to use the concept.
Really, any multiverse cosmology can contain all other cosmologies. The World Axis can contain the great wheel - the outer planes of the wheel are just a different conceptual organization of divine domains, say. The oWoD Umbra can contain both of them, with all the outer planes and alternate prime-materials relegated to the Deep Umbra, and pocket dimensions to paradox realms.
It was integrated much earlier, from the start, as an inaccessible part of the cosmology as all the rest, before Planescape was a thing. The local yokels might not know that their little corner of the multiverse is part of something else hard to access, but TSR made sure to name-drop it as part of the whole anyways. Kind of like a 13th century Tahiti setting wouldn't have access to 13th century Poland, but isn't on a different planet.
Settings or cosmologies are far more fluff than rules, anyway. In the same sense that the GW could be said to subsume other cosmologies, other cosmologies could subsume it.
That's what 5e's good at: leaving it up to the DM.
OK, now I'm going to have to place a weapon called a Fluff Cannon someday...
