I don't know-I think that if she left Harry alive, the temptation to keep going on with the series (or fan demand for it) might be too great-see Anne Rice and her obsession with Lestat for an example.
Good catch with Dumbledore's portrait, W4F.
I've always been fascinated with the living portraits in the series-how is it that a wizard can go on existing after death? And how can they move from one portrait to another and yet still be in all of them, ala Lockhart and his endless self-portraits? Not to mention the famous witch and wizard trading cards!
I don't think DD is dead, and I think the fact that Harry heard a phoenix cry and it made him glad during DD's memorial is a clue to that. After all, a phoenix rises again, right? And IMO, Snape is a double agent for sure. I think he and DD were working not only to bring down Voldemort, but to protect Harry, Draco and the rest of the kids at Hogwart's. DD certainly seemed to be aware of what Draco was being asked to do and gave him lots of opportunity to repent.