I think all Squibs come from pureblood families. Isn't that what a Squib is - a non-magical child born to pureblood parents? Ron described them as the opposite of Muggleborns.
Definitely. The Black Family was mentioned as disowning any Squib members, so there must have been some. From memory, Marius Black was the name of one of these Squibs.
I think we can conclude from the Bücher that as much as they like to think they are, purebloods aren't actually magically-superior in the slightest.
