Other mistake: Despite taking place roughly 30 years after "X-Men: First Class," Xavier and Magneto have hardly even aged. Both would have been in their late 20's at the youngest in "First Class" and in this film neither looks any older than 40. It's not because of their mutant powers either - only Wolverine and Mystique were specifically mentioned as having a slower rate of aging because of their powers.
Other mistake: During the standoff between the aliens and the mutants on the train, Jones tells the mutants to stand down, to which Beast defiantly replies with "No." When he says this, you can hear Nicholas Hoult's English accent slip through.
Answer: Your guess is correct. Lawrence stated she was done with the franchise after "X-Men: Apocalypse." She jokingly told Simon Kinberg she would return for this film if he directed it, and to her surprise, he was serious about it and held her to it. Fans weren't very keen on her appearances either, because she was definitely phoning it in for the third film, so that may have factored into killing her off early as well.
Phaneron ★
More than one actress had played Mystique. Just because Jennifer Lawrence did not want to continue playing that role is not a reason the character would be killed off. They could easily recast another notable actress in the part. I would not be surprised if Mystique is miraculously "resurrected" in a future X-Men film with a new actress.
raywest ★
Aside from Deadpool, any new X-Men film would be a complete overhaul of the franchise since the characters will be part of the MCU now. You're not wrong about an actor wanting out to not really be a reason to kill off a character, but it doesn't preclude them from doing so either. They might have decided it wouldn't be worth the hassle of recasting the role with all the negative baggage that would come with it.
Phaneron ★
Here are some candidates I think could do it: Amanda Seyfried, Shailene Woodley, Chloe Grace Moretz. Heck maybe even Milla Jovovich-Anderson should be given an audition, what say you guys?
Rob245