Is a Lich's natural resist-magic based on stats (such as a combination of int and pie or just int or something else) or is it based on the level you are at and is predeteremined for everyone?

Level based. Their spell r-m is still int-pie I believe.

The way it used to be, it was int+con for Liches, and int+pie for everybody else.

Oh, then it's probably still int+con, since I don't remember it ever changing. I'm too lazy to go search for it in the code.