OK, I read through most of these pages, but after fininshing the book, I'm gonna go ahead and skip a couple; I hope I'm not repeating anything:
I don't think Dumbledore is gone for good; even if he's just stuck in a portrait, he'll be helping Harry out in the next book.
And it wouldn't surprise me at all if Snape's killing DD is a ploy to hid DD for more freedom to work unseen. I think that DD's hand, which he never did fully explain, other than to mention the same curse that ended up "killing" him, had a part to play in it.
So for Snape, he's my theory, hear it out before you judge; Snape considers Harry the son he never had. It has been revealed over the course of several books that:
1: Snape was smitten/in love with Lily (Evans) Potter all the way back in school; but resented her boyfriend/husband. He was, also, envious of their good fortune; good grades, Quidditch player, James' full blood heritage, and seeming prosperity.
2: We found out in this book that Snape was responsible for Voldy learning about the prophesy, and then killing Lily and James Potter, and ruining Harry's life.
3: The info DD had on Snape was fact 1 and 2. That after Voldy killed Lily and James, and marked Harry, Snape went back to DD, and confessed everything; his love for Lily, his devastation at causing her (and James') death, and leaving Harry an orphan.
4: This leads to DD giving Snape a post at the school, both so he can keep an eye on Snape, adn to give Snape the chance to instruct Harry once he gets there; that's why he never gave Snape the DADA job; even DD thought it was cursed.
5: If you look at all of Snapes' and Harry's interactions throughout the books, you can see how hard Snape pushes Harry, much harder than any other student; he is mmost cross when it seems Harry is not taking instruction as seriously as Snape knows he must; Snape knows better than anyone at Hogwarts how dangerous Voldy is, and how well Harry must be prepared; remember, Snaep knew of the prophesy too.
BAM
It all leads to Snape having Harry's best interests at heart through out the series, but not being able to show it without confessing his love and remorse for Harry's parents.
And the last book in the series concludes with Snape's ulitmate sacrifice to save Harry, allowing him to destroy Voldy; while Snape hangs on long enough to give his final confession to Harry, redeeming him in the eyes of both Harry, and the reader.