It does make sense that petunia might be a squib, in some ways, but think about all the times lily was called a mudblood, and didn't deny it. Maybe petunia is just plain jelous. Dudley looks waaaay too much like Vernon to be adopted. And dumbledore is a possibility for death, but come on now, once someone has come so close to death that many times, you'll never buy it when they go.
I don't think Sirius will come back, it was way to legit and thorough, even to dumbledore.
Also, anyone could hold the prophecy once harry picked it up, even the death eaters could, as was said in the book, so Neville is not as important as would be thought, but death is unlikely for him.