^ I suppose I feel the opposite about Alley's portrayal in TWOK. I've never really liked the half Romulan aspect myself, even though I understand the reasoning for it conceptually, and I'm glad it was omitted for the final cut. It's true that her Saavik was more inclined to show emotion on occasion, but I don't see her as breaking the Vulcan rules regarding such displays. Aside from shedding tears at Spock's funeral, she's pretty contained and her chemistry with the rest of the cast works better than the way Curtis was directed in TSFS.
I agree with your second paragraph though. I don't think Saavik would have worked in Valeris' place as the traitor in TUC, because I can't see her throwing away her career and relationships simply to get vengeance on the Klingons for David's death. I'm not sure she would do that even if one counts Romulan heritage.
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.