After I finished reading the book, I was sure Dumbledore was dead, but then, after to reading various comments, I went back and re-read the last section starting with Dumbledore's conversation with Malfoy. He promises that he (DD) could hide Malfoy so Voldemort could never find him and kill him. He says something like, "he can't kill you if you are already dead." Also, did you notice that Snape prevents the other Death Eaters from doing serious harm to Harry. I think there's a possibility that DD paralyzed Harry knowing to prevent Harry from harming EITHER Malfoy OR Snape. I also think there's a possiblity that the pleading to Snape was to go ahead and "kill" him. Interesting that the Ave Kedavra (I'm too lazy to look up the spelling) blasted Dumbledore over the ramparts, instead of leaving him dead on the spot. Also, a phoenix arises from Dumbledore's body and the flames, just before the tomb materializes. Perhaps it is not only Fawkes who can rise from the ashes?
Also, remember about the Potions book. It belonged to Snape, but the book itself was 50 years old, which was when Voldemort was at Hogwarts, not when Snape, Sirius. Lupin, and Harry's father were there. If the book was indeed Voldemort's, then perhaps Snape is the best equiped of all to help Harry defeat Voldemort (provided that Snape is still play-acting at being a death eater.)
With the unbreakable vow, if Snape did not kill Dumbledore, he himself would die. Also, why would Snape help Dumbledore after he was harmed by the curse on Marvolo's ring? Couldn't he have said that the curse was beyond his powers?
When I read the book the first time, I was convinced that Snape was indeed a Death Eater and had fooled everyone, but now I'm thinking that perhaps the person who has fooled everyone is JK Rowling.
Just random thoughts. I do agree that Harry has to "go it alone", and I suspect he's going to be confronted with some terribly choices. This is not a "they lived happily ever after" kind of series.