I still don't believe that he is Alduin and Alduin could be some dark aspect created when Nirn was created. We will find out in Skyrim. For all we know, Alduin could be Peryite, we just don't know.
We really, really do. And if it helps, then yes, do think of Alduin as an aspect of Akatosh. Because taking part in creation took a terrible toll on the Aedric et'ada and rendered them subject to mortal ideas. Listen, time was not linear before Akatosh imposed linear time on young Nirn. Therefore, different races had different experiences and view of different spirits. This enabled multiple realities to exist in the same place. When Akatosh made time suddenly linear, all these competing ideas had to fit within one timeline. So suddenly there was more than one dragon. Alduin isn't a daedra, and especially not one so boring as Peryit, don't be silly. And if you find yourself thinking too hard, realize that Alduin, Auriel and Akatosh all sound similar, are all dragons, are all gods of time. Just accept the obvious.
The only bit of lore that even ties Alduin to Akatosh is the loose association between the two by a mortal writer trying to make ties between the pantheons of the different races. The writer even points out that Alduin and Akatosh have nothing in common except the fact that they are dragons.
Nothing in common! That's because they are opposites, representatives of the opposite spiritual worldviews of men and elves. You're really just not accomplished or widely-read enough to go around saying what is or isn't in lore. There is an entire field of study for this sort of thing, and it is your business if you want to hamstring yourself by ignoring the most helpful texts, but then you should include disclaimers when for the benefits of this young and impressionable forum.
They weren't avatars, they were reincarnations, big difference.
Big difference, maybe, but not really so important. Lorkhan has had a hell of a lot more impact on the mortal world than Dibella recently. And that should get him a more nuanced characterization than 'dead.'
Akatosh didn't invent time. He was the first spirit born from Anu, thus he found it to be his charge to watch over time as he was the first spirit. Time was not his creation.
Except for the myths that say he did invent time, and that it helped impose a bit of order on the universe already teeming with spirits. You can't just grab a text of the shelf and take it literally. We can only know what happened 'back then' by examining the commonalities between various myths. And one of those myths will tell you that the Aedra are not purely Anuic, which should be obvious because purely Anuic things are simply ball of inert energy that can't do anything at all.
My point exactly, the nine have never physically manifested themselves on Nirn since the Dawn Era. People need to realize that the nine are not like typical gods, they LOST their immortality. When they die, they don't do what normal gods do, they don't move to the god state where they repair themselves and resurrect, if they die, their spirits move into the dreamsleeve, thus meaning they died. That is why they were pissed enough at Lorkhan to kill him. The main reason the nine don't affect the world is because they are agents of Stasis, they try to preserve what is and the Daedric Princes meddle in the world constantly because they are agents of change.
If a god can't manifest physically (and the Nine have sent down physical avatars), how the heck do you kill it? No Aedric Divine has ever been killed, except for the sort of death they all died when they sacrificed themselves to create Nirn, so who knows how exactly it would work.
And you're being very unfair to the Aedra. How do you get off calling them agents of stasis when they took part in the most radical, unprecedented, impossible endeavor the space-time continuum has ever seen, the creation of Nirn? Where was the stasis in that? Now that the world is here and they are bound to it, of course they will try to preserve it. They will even try to preserve it against the rogue aspect of Time deciding that Time Is Up.