Mystique was less sure about this than Magnus. Entertainment value or not, these were risky people to do business with. Namor had only his own interests in mind, certainly not the good of all mutants, and the woman...well, besides being human, she was so twisted in history it was hard to say who's side she was on from week to week.
Still, if this was what Magnus wanted, she'd be there. At the very least to make sure no one tried to stab with with the butter knife.