I think Snape is still on the side of the angels. When Dumbledore was pleading with him, it wasn't "Please don't kill me, Severus" pleading. It was "Kill me so that Draco doesn't become irretrievably lost to evil, I know you people can't leave while I still live." And Severus could have done far worse to Harry in parting without worrying about depriving Voldemort of his rights. "The Master wants him alive" is classic double-agent speak for "I want him alive but blowing my cover now would probably get me killed by BOTH sides."
Oh, and for the record: I guessed right about Dumbledore dying, guessed wrong about the ID of the Half-Blood Prince (I was sure Riddle was the obvious red herring, but thought it might be Dumbledore), and was totally off the mark on Tonks (I had her pegged for a "and years later, Harry and Tonks finally found each other and were married" epilogue, although I suppose that's not impossible). Kinda disappointed Harry did the stupid Peter Parker at the end of the Spider-Man movie thing, although I doubt she'll let him keep her at arm's length for long.