I think its fun to speculate even if I'm wrong
I agree with you so much about this. I've read around about how Chris Metzen et al have fleshed out the Diablo lore over the last few years. I really enjoyed the slightly vague suggestions offered in the first two games but now they've gone right back to the beginning, altered things around and even gone so far as to provide a creation myth.
I realise a lot of people prefer to be spoon-fed the story and not have to try too hard to find all the pieces. For me this leaves nothing to the imagination. Having followed - and played - StarCraft and WarCraft games since 1996 (Diablo being the game that got me interested in Blizzard of course, so I came to WC2 a year late) I'm sure they'll give us a good story but I have to say I much preferred the Dante's 'Divine Comedy' aspect of the Diablo world to the sweeping mythos of WarCraft. That doesn't mean I don't like the WarCraft lore, because I do. I just felt Diablo had been left so vague it was by far the more mysterious.
For many reasons, speculating on anything at the moment is very difficult because we don't know what aspects of the established lore they are keeping and which have been retconned. So we don't know how they are tidying up the inconsistencies between the first two games. We also don't know how they are filling in some of the more vague aspects of the first two games. A lot of the content in the beta is a huge homage to the first game, with a few references to the second. It feels to me as though they are taking Diablo 3 right back to the beginning to bring it all together.
Okay, that was a bit of a rant I admit. Sorry about that! :P
In regards to Tyrael, it would make sense he would become tainted. We've seen how the heroes from Diablo became corrupted just by facing all the evil in the labyrinth beneath the Cathedral. They only battled Diablo, a single Prime Evil. Tyrael was confined briefly with not one but two Prime Evils, in the Tomb of Tal Rasha. He did not bow to them and I would imagine that if the war between heaven and hell has been raging for eternity, he has likely faced both of those brothers before. If he wasn't corrupted already, I don't see why that one encounter would make any difference.
If you pay attention to things Chris Metzen says, it seems obvious he likes there to be some hope in these stories. He believes it to be essential for heroes. I suspect Tyrael will be that hope, at least for the time-being. I doubt they would corrupt Tyrael that easily but I could be wrong.
So for these reason I think no: Tyrael is still our ally and a hero. I think he might have been fooled at some point, probably more than once. He does seem to be the only force for the heavens we've seen. He has helped us contend with not just the destruction, hatred and terror of the three Primes but has helped us overcome anguish and pain too. Now we have to face off against lies and sin and likely the return of terror. If he could do it alone, heaven would have won this war a long time ago. He's not perfect but he is not evil and he is not corrupted. At least, that's what makes sense to me. :P
I shall finish as quickly as I can. I've blathered too long already!
The warrior headed east after he was 'comforted' (nice euphemism). It seems to me that Adria has followed in his footsteps, at least part of the way. She seems to have taken another character from Tristram along with her. That remains to be seen, though.
Consider: Izual said he helped the three Prime Evils mastermind their exile to Sanctuary. Ask yourself: would the three Prime Evils inform the Lesser Evils of their plan? Would Andariel, Duriel, Belial and Azmodan know exactly what the three brothers had been up to?
If the answer is no, the Prime Evils would have kept it to them selves, rewatch the Black Soulstone cinematic and wonder on the implications of who Azmodan thinks he is talking to.
If the answer is yes, the Black Soulstone cinematic takes on an entirely different meaning.
I suspect the answer is no: the Prime Evils kept their plan to themselves. So which trap is Azmodan talking about? The Soulstones? Seems likely, considering he mentions the never-before-heard-of Black Soulstone.
As to Leah or Adria being Diablo: not trying to be too Agatha Christie here but they both seem like red herrings to me. They're just a little too obvious and too many fingers are pointing in their direction. From the first game it seems Adria had lots of visions. What do those two recent cinematics we've seen suggest about Leah?
As for the heroes from Diablo 2: they faced off against all three Prime Evils and two of the Lesser Evils. Did they ever really stand a chance? :P