Magneto and Mystique did not join the team. The worked along side them because they had ulterior motives. There was no redemption there.

I highly doubt that they would have Jean become evil and then back to being a good girl at the very end. That would be demeaning and just plain dumb. She could come to her senses, but actions have consequences and she will more than likely have to pay for her actions even if it is by her own hand.

As far as the Shiar thing, that is fine, they would not work for the film. However, showing Jean Grey with some pretty shnazzy beefed up powers and her gradual seduction to the darkside would be far more entertaining. Instead,, you have her return and immediate turn towards the darkside. So rushed.

They only redhead we're going to see living and breathing at the end of the film will probably be Syrin. Though she leans more towards the orange side of the sprectrum.