I don't think Petunia's a squib. Rowling goes to great lengths to make it clear that Lily is muggle-born.
Could it be that Petunia is also a witch? And that Lily's powers were stronger?
In the early books it was always the Uncle who freaked out when Harry did things. Maybe it's just the way it was written but Petunia didn't seem especially suprised when Harry's hair grew back overnight.
Good point. But Petunia's knowledge of dementors and Askaban was obviously intentional in OOTP. How did she learn that. I can't imagine her asking Lily.