So you are of the belief that there is no single truth in TES. There is no correct Akatosh in that Alduin, Auri-el, Ruptga, and Alkosh are all autonomous beings, so Auri-el and Alduin could meet and have a conversation with each other. You could say that I'm stuck in the mundane, single-truth mindset, and I think that there is one Akatosh that is seen differently through different cultural lenses. I feel like there is a single narrative that actually happened, and only through examining all the different cultural interpretations of that narrative and taking the bits of overlap we can weave it all together to find out the truth.
Yes, Alduin might certainly speak to Auri-el. The primary attribute that characterizes aedra, whether they're gods, mer, men, etc. is that they're magicka/creatia/anu-stuff subject to the division of padomay/PSJJJJ. Bits of it can be an aedroth on Nirn, experience a unique perspective and communicate with other such bits. What separates gods from mortals is that they are not limited to one life or story. Lesser aedroth are creatures of a local physical body that takes a singular path through time and consequence. The Aedra/Aedroth that is the earth-bones is not limited to singularity, locality, or even contradictory exclusion--If it can be 8/9 divines all at the same time it can certainly be multiple versions of each of them simultaneously as well. What mythopeia does is help define those distinctions and interactions. The overlaps in the narratives are important, but not to the exclusion of the importance of their differences and unique distinctions.
The reason there are multiple narratives is there are multiple cultures who obviously see things differently. If by making a myth a culture creates a god, whose to say that a person can't invent a god thus willing it into existence.
The earth-bones are HUGE. They're everything. They're Lorkhan's Corpse and Nirn and all the Planets. They're everything and everyone and every god. They are the ontological reality of Mundus. Every god-story that achieves any sort of notoriety and relevance will align to them. It's very important to remember that Nirn is a world where the physics are made out of magick and that stories about gods have real power. That's the whole bit that makes mantling different from incarnation. Incarnation is when a god (story about the earth-bones) has enough personality and power to walk among mortals with an avatar. This is great for helping to make new stories and alter cultural interpretations; mythopeia is a two-way street on Nirn. Mantling is when you re-create the narrative elements that define the personality of a god in your own life, at least in the popular perception. Once you've done that, the story of what you do next becomes a new story about the god by default. These can be combined, and that's what passes for politics at some level.
If everyone on Nirn suddenly forgot that gods existed I don't think the gods would cease to exist. If that were the case, the lent-bones would cease to exist as well, and since the gods are all part of the Godhead, the godhead would cease to exist as well. What if one person decided the gods weren't real? Would a single person's worth of god-material vanish? And what of the Dwemer who believed the gods weren't even gods?
The reality of magick on Nirn isn't dependent upon mortal belief for its existence, only its shape. A lot of what you believe about how much the lent-bones depend upon the support of the gods might depend on what you believe about Talos, but the Godhead definitely does not rely on the Aurbis for its ontological support--that is strictly a one-way relationship. The godhead derives its teleological support from the AE, which is the most essential point when you get right down to it.
One facet of the discussion here is that Nirn is a world with demonstrable gods. One person's disbelief isn't going to have an impact, and Nirn has a net magick surplus, what with all the stuff flowing in from the sun and the stars and trafficked through Daedric channels. And what of the Dwemer? Perhaps you should ask them about the ultimate consequences of their theology.
In addition what of the creation feedback that exists when mortals on Nirn use myths to give power to gods and create their own? Godhead splits and fractures into multiple gods > Aedra create Nirn > mortals on Nirn create gods [feedback]. That's an infinite creation loop that could only be pinched off at the end of a kalpa.
The god-shapes formed by mythopeia exist within the context of the Mundus and the power of the earth-bones. You don't get new sub-munduses every time a new god-concept rises or an existing god-concept gets modified. Well, not necessarily. There are certainly ontological disruptions associated with radical alterations to the gods. However, it seems that the next sub-gradient "down" from the Mundus has to do with CHIM, which I am not at all prepared to address.
If such a model was real, the whole of the Aurbis would look like the image through a kaleidoscope while underwater and dizzy. So I'm of the realistic school, the mundane school call it boring, but I like to have truth with my toast in the morning.
This perspective doesn't work external to the Mundus. The limit of Lorkhan is what reduces the Mundus to a single truth. Outside or before this, the Aurbis is a realm of infinite potential. Every metaphor for creation is present in the Dawn.
Don't despair, though; the abstractions that these metaphors point to (truth?) are also present in the Dawn. Aka's crazy because his shadow took a peek outside the Aurbis, and saw (among other things) the infinite isolation and imprisonment of the Godhead. Lorkhan is Aka's knowledge that the Aurbis does not have meaning imposed from without, and worse, that it doesn't generate any of its own from within. Everything exists as raw possibility, but nothing happens and nothing matters. Lorkhan wouldn't let Aka forget that he was the Godhead, so he Zero-Summed.