That's interesting, speaking as a non-fan of the X-men, merely of the movies, which have gotten increasingly intolerable, and more absorbed in the characters, with a presumption that you already like them. To me, Xavier is some rich entitled asshole who has way too much power and meddles with things that are none of his business, in addition to approaching a highly implausible social problem in just about the worst possible manner. Xavier has no more right to freeze other people's perception of time, than the government has to put mutants in concentration camps or deploy homicidal robots to eliminate them. The films waffle between a disagreement as to how a bunch of superpowered individuals are managing their more-white-than-white-people-problems, and making absolute statements about the rightness or wrongness of the whole mutant question itself. Magneto's whole anti-human ideology is so over-the-top cartoon villain, he is hard to take seriously, especially when as the X-men's supposed arch-foe, he is allied to them in like five of the seven movies in which he appears.

Part of this might be my indifference to rewatching the older trilogy, so I might have the details wrong, but I really don't see any coherent character arc for Mystique. You provide the best rationalization for the Jennifer Lawrence character, but damned if I can see how she gets from there to Rebecca Romijn's character.