To be honest, I have doubts that Alley would have been contracted for multiple pictures in 1982, since not even the "regulars" were contracted for multiple pictures. It doesn't make sense that she'd be optioned for another film when other actors were renegotiating their contracts film by film.The issue, though, is that her contract did not require her to participate in the sequel for a predetermined price which, as I understand it, is fairly standard language in the contract for a newcomer.

Valeris works for the role because she embodies the fear that the Klingons simply can't be negotiated with, they're "animals" as Kirk says in the beginning. The enmity between governments is so old she can't imagine them being anything else.