I didn't believe that Diablo became Tathamet in the first place. I don't see this as possible within the lore. Following his death, the seven heads of Tathamet spawned the Great Evils. His headless corpse became the Burning Hells. I don't see how Tathamet could become whole again. If Diablo did transform into Tathamet, wouldn't he have been unstoppable? Wouldn't Sanctuary have burned? I don't think that anyone but Anu himself would be capable of stopping Tathamet. Plus, if Diablo did become Tathamet during the events of Diablo III, I'm sure that they wouldn't have left us guessing. Shit would have been explicitly stated during the narrative of Act IV.
Yeah that's one thing that they did kind of strangely, hyping Diablo as the Prime Evil and this super mega badass. On other forums, I've seen people saying things like 'lol how can there be an expansion, we killed the prime evil' 'yeah right, we killed the prime evil and now we gonna fight the sin lieutenants or something?'
They should have done a better job of explaining what exactly the Prime Evil was in context of Diablo's 'prime evil' aka all the evils in his body. (Or Leah's body I guess technically ) Especially since they have Tyrael randomly mentioning Tathamet (which like you said, should have roflstomped whatever went against him, unless we are to believe one Nephalem is that powerful) when asked what the Prime Evil refers to. I guess that may be intentional though, as it seems 'prime evil' seems to refer to a few different things, like in that quote from one of the devs the 7 Evils are all 'prime evils,' the 3 brothers are also the 'prime evils,' then of course the 7 Evils in one body is the 'prime evil' and Tathamet was the original Prime Evil.
I don't think that all seven of the Great Evils are considered Prime Evils. Just Mephisto, Diablo, and Baal. Tathamet was the original Prime Evil, and Diablo became the Prime Evil. You're right, this is confusing! Tyrael mentioning Tathamet doesn't help either. I think that they could have done a better job explaining this within the narrative. Regardless, I am very impressed with the story of Diablo III and the way that it's told. I am sure that the first expansion will elaborate on everything that has happened regarding Adria and the Black Soulstone. Also, I think that both you and I know for certain that Diablo did not become Tathamet.