Maggie Q's starring role on Nikita is most likely what precluded her return.
This was my assumption as well. It could also be the case that the producers wanted a different actress and/or female lead in the film. Beyond Michelle Monaghan, has any female character appeared in more than one Mission: Impossible
film? Or, it could be that some genius somewhere was worried that if Maggie Q came back, the unwashed masses would mistake her for her Nikita
character in the film. Whaddyagonnado?
Anyway, it's interesting that Jane has the same last name as the originator of the IMF team role she's filling, Cinnamon Carter. I wonder if she could be Cinnamon's daughter, but that would mean that Cinnamon ended up with an African-American husband (or whatever) rather than with Rollin Hand as one would've expected.
Not necessarily. She could just as easily have been adopted. Or.... be from a different Carter family.