1. from the begining of the book, when snape is extracting information from narcissa about draco's quest, he asks bella, do you really think i'd be able to fool the dark lord? yes. he would. because if dumbledore was really the best at occlumency and legilimancy, he would have been powerful enough to teach harry himself. the thing about great leaders, is their ability to know their weaknesses as well as their strengths. snape is the best of the best when it comes to those things and that's why dumbledore had him teach harry. so yes, snape would have been able to fool the dark lord.
2. dumbledore has never allowed snape to be the defense against the dark arts professor. he still hasn't. perhaps that was dumbledore who was teaching DADA (as snape). the position has been cursed, as we know, and a teacher has never taught it for more than a year. that is all he dumbledore needed to teach it for. he still wouldn't let snape do it - he taught it himself.
3. from the moment i started reading, i thought dumbledore was acting strange. it continued throughout the entire book. he is usually quiet and observes everything and now all of a sudden he's keen to let harry in on everything? and why is it he keeps telling him to tell ron and hermione. maybe because snape thinks harry wouldn't be able to figure things out himself? and "dumbledore" even loses his temper a few times, nothing major of course, but he snaps at him - that's snape coming out. dumbledore is just acting odd. what was especially weird was the fact that harry had to get that memory from slughorn...
4. also in the end at the cave, when dumbledore drinks the potion and screams, i'm sorry and all that, that could be snape saying he was sorry for ratting out james and lilly. that is simply too weak for dumbledore.
5. on the astronomy tower when "dumbledore" tells draco he can protect him and his family, he's talking to him very personally, very lovingly. he can protect his mother, his father is safe. it makes more sense that is was snape saying this. snape is the one that is close with the malfoy's, not dumbledore.
6. in the end, when professor mcgonnelgal is talking to harry, she says 'professor dumbledore told us snape would be out of the castle for a few hours.' well of course he would, he was in the cave with harry! and dumbledore knew it, of course.
7. harry has been checking the m. map, but never once does he mention seeing dumbledore or snape. he's only concentrating on malfoy. usually, when he looks at the map, he sees snape in the dungeon or dumbledore pacing in his office. if he had seen this, he'd have known.
8. there's something fishy about snape being "prince" and voldemort being "lord." yes, this could possibly mean that snape is next in command for the dark side or something, but i think snape is still good.
as far as snape or dumbledore being dead, i am not sure. i just think there were some weird things about snape and dumbledore in this book.
also, slughorn telling harry that he's just like his mother in potions? he told him every single time. something's going to happen with that for sure. what's his mother got to do with snape? i know, i've read that theory about snape being in love with her... who knows...