Wouldn't Laura and Cynthia be too old for Karen and Martha?
While I can understand that Linney may have been very wrong for Merteuil (I didn't see it), (and while, unlike BobbyBubbi, I love her work generally, and particularly in The Crucible), I would suggest that Merteuil and Regina, for all their villainy, are hardly the same character.
More to the point, Regina is much more like the buttoned up, status-consciously conniving and nothing if not American Bertha Dorset in House of Mirth, a part that Linney aced in the great Terence Davies-Gillian Anderson film.