http://www.imperial-library.info/content/oblivion-lunar-lorkhan quickly stats that the eight gave their power to create the mortal plane. It was a trap, depending who you ask, by Doom Drum. Doom Drum convinced the aedra to create the mortal plane, and that some of their power was needed to do so. Not soon after, many learned that the mortal plane was taking more of their energy that what was suppose to. Some excaped, creating the sun and stars that we see. Other's died - becoming the Earth-bones. And the last eight became the divines, Akatosh included.
It was my understanding that all the ones that sacrificed themselves were Earth-bones, but some stopped half way through and used faith to keep themselves together and alive. The Nords see Aldiun as both a creater and a distroyer as said http://www.imperial-library.info/content/morrowind-varieties-faith-empire, not just an eater. That's why I think that they are the same being, with the mortals just looking at the different things he did. If the Nords see Alduin as both a creater and a distroyer, then Aldiun isn't just the evil aspect of Akatosh.
Yeah, that's mostly it. The being now known as Akatosh was trapped on Nirn, presumably because he didn't read Lorkhan's fine print. The Earthbone spirits were subsumed completely, becoming the physical substance of Nirn.
But we can also describe the sacrifice of the eight et'Ada spirits as deaths. They gave up part of themselves, and their immortality with it. What does this mean functionally speaking?
They became, for all intents and purposes, dead planets stuck in Nirns orbit. The original eight 'died,' by losing their power and identities. But their mortal descendents re-constituted them based on their memories of the Dawn Era's struggles. Since the elves and humans had different experiences, they remembered different deities. Different deities were therefore constructed from their opposing beliefs, and they are all very, very real.
Auriel wants to escape the mortal world, as per Aldmeri belief. Alduin wants to destroy it. Akatosh wants to preserve it, and not just as some stalling tactic so he can pull off his own escapist coup. He is a being that has been completely reinvented by mortals, and should be viewed as separate from Auriel and Alduin, and
completely separate from the original spirit whose corpse floats in the sky as a planet.
Alduin is a renewer deity who causes beginnings by bringing about the end. He reflects Skyrim's cyclical conception of time (which their southern cousins do not apparently share) and their dread of time's ravages on mortal works. And really, he is just Auriel as imagined by a hostile Nord. But Auriel still isn't a dragon, and has a mind of his own. Basically, we are dealing with an exclusively Nordic view of the apocalypse.
Akatosh is a statist deity who reflects Cyrodiil's confidence and self regard. He fortifies Nirn just as he fortifies the Empire. He is really almost Alduin's antithesis, forged as he was from an awkward compromise of pantheons. Maybe that's why he's insane. In any case, he's not plotting on behalf of his dragon twin. He doesn't share Alduin's goals and is incapable of acting in concert with his alter-ego.