There just isn't enough evidence to positively conclude that and it seems a bit odd that we would already know the main plot of who the enemy is before we even get close to the game. I'm sure Alduin and Akatosh are completely different beings.
Missing that Akatosh & Alduin are the same guy is like missing the "Big Brother is Watching You" in
1984.It's a pattern of myth that you're meant to draw lines between, and while there are some areas where it gets hard to draw these lines, this is one area where it's really easy. This pattern of myth is the literal foundation of the world of Tamriel. If Alduin is not Akatosh then Tamriel doesn't exist.
If people can go around saying "Alduin is Akatosh and he will destroy the world" and have all this backstory that isn't established as lore to explain him I can say that I don't believe he really is Akatosh. I don't see why you guys think you have to right to say something as fact that isn't fact but me saying that I don't think he is and not stating it as fact is such a terrible thing. Also, he isn't literally "eating the world". Consuming the world is a metaphor for destroying it. You can't take TES lore completely at face value, you have to realize in what context it means.
Since you seem sorta new and unfamiliar with the lore forum and the lore in general, I'll go ahead and save you the trouble of searching around for answers:
http://www.gamesas.com/index.php?/topic/1158326-how-does-one-eat-the-world/page__st__20__p__17010120__hl__alduin__fromsearch__1#entry17010120
Really the first and second lines are all you need; precisely the
"it's going to be the whole of Nirn inside his mighty gullet." which is about as literal as it gets when it comes to whether it will be him literally "eating the world." http://www.imperial-library.info/content/seven-fights-aldudagga will give you the same thing as will http://www.imperial-library.info/content/morrowind-varieties-faith-empire and http://www.imperial-library.info/content/oblivion-five-songs-king-wulfharth-1 Nords aren't known for their deep thinking - if they say something eats the world, they mean it's literally gonna
eat the world.
That's a misconception. Auriel and Akatosh weren't two separate beings, it's just Akatosh with a different name to the elves. It's not like Akatosh and Auriel walked up to each other and said "Hi" and hugged each other. The difference here is that Auriel and Akatosh are the same being and they also have the same aspects. Alduin is nothing like Auriel and Akatosh and it also adds to the effect that it took everything out of the Aedra to make Mundus, how could they destroy Nirn and rebuild it as they please. Why would Akatosh kill off the worshippers that give him and the other eight divines their power? Seems to be against his own interests. Also it goes against the very fabric of his being. He is the first spirit of Anu, the force of Stasis, meaning keeping things the same, thus is why he and the other aedra haven't directly manifested themselves on Nirn since the dawn era, the daedra are the ones that continually meddle with the world. The only evidence that Akatosh and Alduin are even related is a book that loosely associates them, otherwise they don't have anything in common and Alduin is everything that is against a spirit of Anu such as Akatosh. Also, I don't want to hear the argument that Akatosh is schizophrenic, because if gods are so omnipotent and powerful as MKs musings suggest, then a schizophrenic god of time would've already shattered time with his insanity.
Time has many aspects, the Imperials revere one aspect of it, the Nords revere another. Both still revere Time. The Aedra don't manifest themselves often because they're good parents. And again, if you think that the gods are ever presented as omnipotent I might suggest taking a look at the pinned threads in this forum, because such is never suggested by anyone anywhere.
When you say "a book that loosely associates them," do you mean the one that says
"Alduin is the Nordic variation of Akatosh..."? Or do you mean the general trend of the entire mythos that suggests the same thing? I would think the fact that they're both referred to as the god of time would be enough...