I think Birthright should be divorced fully from the previous Planescape cosmology - that was actually the Greyhawk cosmology that TSR applied to all their worlds, so they could justify characters crossing over, or whatever.

That said, let's look at what we got - we have the Shadow World, which is a very strong world element, and then there's the brief godly domain descriptions or names in the Book of Priestcraft. I don't think the latter need be taken strictly as is, given that it was written within the context of being forced into an existing system, rather than defining a cosmological system of its own to fit the core game world better.

My view of Birthright cosmology tends to focus on the different natures of the old and new gods. The old gods seem to me to have been more elemental and primal in nature, whereas the new gods are ascended humans, and remain more closely tied to that aspect, yet separate themselves more from human affairs than the old gods did.

The Shadow World currently seems like a natural point to begin - the realm spirits journey through towards their afterlife destinations. Its nature changed after the battle of Deismaar - and prior to the battle of Deismaar, the Shadow World as described then doesn't necessarily strike me as being as much of a world through which the spirits of the dead would journey as much as it seems to be a fairytale land (and I don't tend to equate that with "where the dead journey," but perhaps it could still be used - something like the spirits of the dead would pass through the Shadow World at night, when the fae were hidden).

Beyond the aspect of "the dead pass through the Shadow World," it doesn't necessarily strike me as having been much of a transitive plane as it was before - more like a happy mirror world than anything else. Would it be logical to presume that this changed along with the nature of the plane?

What if the dead in the days of the old gods didn't go on to some heavenly or hellish domain, apart from the greatest heroes and foulest villains, but rather joined with the elemental spirit of the world or whatever, so that their souls would be purified, reincarnated, and born anew or something - something much more similar to what happens to dead elves (which I think was discussed in one of the BR novels - Greatheart or something - not that I hold that as canon at all) and that this changed when the old gods died, and the new gods came into being - the new gods, being closer to men, and less primal in their nature, would open their realms to all dead spirits, and then have them come there, and thus maintain their individuality after their deaths, breaking the old reincarnation cycle - out of pity or sorrow.

In the same vein, the godly realms would seem to logically be tied to some manifestations of the elemental natures of the old gods - that their domains themselves were coexistent or coterminous with Aebrynis at certain points, rather than having to go through the Shadow World to get at them; i.e. in order to cross to Anduiras' domain, you'd have to scale to the top of the highest peaks, in order to get to Masela's domain, you'd have to sail until the ends of the sea. Then, with the coming of the new gods, this was changed, and the gods drew farther from the world, anchoring their domains in the Shadow World instead, and drawing the spirits of the dead through that world. The very nature of the old gods' domains might've been that of pristine, pure nature, while the new gods erected great palaces and cities there, to house the coming of the spirits of the dead. The change in the nature of the Shadow World might thus've been only partially the Cold Rider's doing - perhaps such a cosmological shift is what enabled him to do what he is doing (speculating about speculation, blah).

Finally, angels, devils, demons, and similar outsiders, would be the great heroes and villains of old, those who retained their individuality beyond death while the old gods still lived - each infused with a portion of divine might. They'd have been the greatest servants of the old gods, both in life and in death - this'd set up an interesting niche for these creatures as well, I think - they'd retain knowledge not even the new gods have direct access to. This version of a Cerilian cosmology also seems to lend itself well to an epic campaign - perhaps having spirits live on in the afterlife is weakening the natural world slowly, killing the world itself, and that this must thus come to a tragic end at some point?

I think this is a potentially workable view on the Cerilian cosmology - I definitely don't think the Great Wheel cosmology is appropriate.