I think this thing with Snape is interesting. I am interested to see if by chance he only wanted to save Malfoy, and not actually kill Dum. But I think that if he is on the bad side that there will be no turning back. I think Dum. is gone for good. Possibly Harry will consult his portrait for things, but I don't think he is coming back. It said in the end Harry will stand alone against Vold. Everyone knows that if Dum. was alive he would stand between them. Harry has to do it alone, so I think Dum. is gone for good.