Yeah, I'm in a weird place right now, don't know how I feel about him being human and all. I think the main problem is that his transformation was too early. If it happened near the end of the game, it would have been much more meaningful.
Why would Leah be back? She was just a tool to bring Diablo, along with him - she died. He (Diablo) has the habbit of making his comeback in different ways everytime. Leah most certainly won't be back according to Diablore-logic.