Once again, where did you get this information? I don't remember Alduin being referred to as a god, I remember him being likened to destruction of the world by the nords in the book but nothing else. If you could, could you cite where they said he was a god in game lore. I'm not saying that rudely, I really want to know so that I can correct myself if I'm wrong.
From http://www.imperial-library.info/content/morrowind-varieties-faith-empire, in Alduin's paragraph:
Nords therefore see the god of time as both creator and harbinger of the apocalypse. He is not the chief of the Nordic pantheon (in fact, that pantheon has no chief; see Shor, below) but its wellspring, albeit a grim and frightening one.
Not only do they acknowledge him as the God of Time, they place him within the pantheon (though not at the head, Shor's too cool for that).
In all instances, Auri-El and Akatosh aren't even two separate entities, they are just different names by the Aldmer and the Imperials.
If that were true, understanding separate in terms of facets differentiable in more than just name, then the Selectives would've had no need to dance on their Tower.
Yes, they are alike. Just in Aldmeri lore, Lorkhan is the big bad evil guy who tricked everyone into losing much of their power and severing the Aldmer's ties to the spirit world. Auriel lead an army against Lorkhan's army. If he really wanted the world destroyed (or even had the capability) then he would have already done it. As of now, mortals give the Aedra their power, why the hell kill off your power source? Besides, I'd think the Nine would be happy continuing to pull the strings on their puppets on Nirn. Metaphorically of course.
Why wouldn't Auriel destroy the world? Dunno. Maybe because he was too busy fighting his Mirror-brother(s).
EDIT: Or maybe, seeing as it makes little sense for Time to end at the beginning, the conditions for world-eating weren't yet met.
As to the nine gaining definition through mortals, there'll be mortals again once the dawn's returned. It's not like they're gone forever. Kalpas, remember? We made it to destroy it to make it to destroy it to make it ...
Also, don't forget that Akatosh was the first god, born from Anu, the force of Stasis. So it is within Akatosh's purview to preserve things as they are. If you think about it, the first time that a spirit of Padomay (Lorkhan) convinced him into going outside of his force and fostered change, he regretted it...deeply. Why would he do it again? Not to mention that time is linear and not cyclical.
Because of the enantiomorphic relationship. And also because Time, in its very definition, is change. What does Time do, in its very nature? It allows there to be a unity of continuation by which things no longer remain the same.
EDIT: Or rather, Time, by its very nature, allows for the negation of things locked in stasis.