Really interesting premise. I like this kind of AU, that explores what might have happened had X event gone differently. The way you've woven the AU in with canon elements is fascinating. It has a very "Sliding Doors" sort of feel to it, in the sense that certain events seem inevitable, and it's only a matter of when they'll occur in the scheme of things, and not if.
Also, there's this:
"Would you like to?" Harry asked. "'Cause in my dreams, you loved my mother. And you do, or did, didn't you? You're always looking at my eyes, all the time. It's because I have Mum's eyes."
I want you, you idiot. Snape stood up and, taking Harry by the hand, guided him to his feet as well.
I like that you haven't dismissed canon, yet you've made it clear that, here and now, Snape wants Harry. Period. It's simple yet poignant.