I don't remember a scene that had Charles & Mystique together in any of the three movies.
They didn't have too.
She tried to murder him using Cerebro in X-Men 1.
That's some gratitude for taking her in and caring for her all those years, huh?
I guess this is a case of simply choosing to ignore X3.
They still included Beast & the cure plot from X3 in "First Class".
It seems kind sloppy that they'd ignore a major thing like Charles walking and not the lesser, such as the cure.
that they used the Moira McTaggart name for a role that was completely unlike her character and could have been filled by any number of other Marvel characters.
They could have used Angel's old gf from the comics: Officer Jones and just made her FBI instead.