How can a horcrux be a person?
There wasn't anything that said it couldn't be, as far as I can remember.
But if a horcrux is a piece of someones soul, then if that piece of soul resides within Harry he would currently possessed by voldemort. much like the piece of soul that was in the diary that possessed Ginny in the 2nd book.
But you're forgetting Harry's family protection, the reason he has to go back to his Aunt and Uncle's house every summer. That could be the difference. Maybe he's the perfect horcrux for that reason. That would also explain the parseltongue. I don't know that I completely buy into the "Horcrux Harry" theory 100% but I think it's very possible.
Also i do think snape loved Lily and thats why dumbledore trusted him so much...whats the consensus on whether or not snape is really evil? I think he is and if he wasn't Rowling would've summed it up in this book instead of letting you wonder. Also the only way to show that snape isn't evil is to have Harry trust him at some point...which he has never done. I only foresee that happening if Snape is saving Harry's life...which would be interesting.
Again, I say that Snape has to be ultimately good for Rowling to have made us give a shit about him at all. He may be an asshole to Harry but he's never been painted in the same pure evil way that the Malfoys or any of the Death Eaters are. But he'd be irredeemable if he killed Dumbledore in cold blood without there being an understanding between the two and, as RegularKarate said:
Dumbledore trusted Snape and I don't see Rowling's lesson being "you can't trust people"
The real test of Snape's loyalty will come if and when he has to choose between saving Harry or Draco. Does he sacrifice himself for Harry and the greater good or does he fulfill his unbreakable vow and save Draco (and himself)?
As for Snape being in love with Lily... it's probably the case but something doesn't sound right to me about that. Can't put my finger on it. Maybe that it hasn't been established already (has it?).
God i can't belive i've posted more in this forum than any reasonable ones..