so is supposed to be Loki. And i remember that alos Carnage and Sabretooth were at times said to be immune to mind control that with symbiosis and wolverine-claw-to-the-head thing.
BUT
A possilbe explanation could be:
a) it was't just a psionic attack but a form of exorcism made up on the moment combining different types of magic to affect a powerfull purely psychic entity like it is onslaught(remember in the old times not even ALL the heroes of MU werent able to do much against him and mutants and reality warpers just made him more powerful. Only Chaos magic seemed to work a little)
b) The inversion is not as complete change as much as an axis change. Loki is still Loki and seem to be the only one realizing something is wrong, and Sabretooth seem more like his AoA coutnerpart in behaviour(doubt he will be allowed to redeem himself...fandom is cruel superstitious lot..). So Mystique could feel more compelled to act like a decent person but still be Mistique....After all there ha dbeen times where she acted like a more three dimensional character rather than the flat psychovillain she had been portraied in latest years.