The only true viable evidence is that Ultimecia uses Squall's GF, Griever. Otherwise everything else is a bit iffy aside from the fact that Squall is there every time any of the primary Sorceresses hand off their powers to another... and In going to the time-compression/future Squall (and party) had to eliminate a chain of Future/past sorceresses that all of which would have already been absorbed in the future by Ultimecia.
As for everyone elses parents, you only run into Squall and Rinoa's birth parents. General Caraway and Julia = Rinoa and Laguna and Raine = Squall. I always found it interesting that both Julia and Raine died before the actual start of the game.
We know we run into Zell's Adoptive mom... but I'm not sure you run into everyone elses parents (adoptive or not) throughout the story. Also, Quistis states that she didn't take well to her adoptive parents and went to live back at Balamb Garden and basically be raised by Cid. *shrug*