"Stunt casting refers to someone being cast in a role they're not right for in any way."
I don't think that is the defintion of stunt casting at all. Unknown actors may be cast in roles that are not "right for in any way." And same with famous actors. it happens all the time. But that has nothing to do with "stunt casting."
When used in reference to Broadway shows, stunt casting is a term in casting that refers to hiring of well-known TV or film actors to play small roles in an attemtpt to generate a "buzz" for the show. It is different that "star casting", which refers to casting similar individuals to play major roles. Thus, casting Catherine Zeta Jones in A Little night Music was an example of star casting, not stunt casting. In the case at hand, KeKe falls into the star casting defintion more aptly, while Sherri, who really is playing a relatively small role, is more "stunt casting." Similarly, the casting of Carly Rae was "star casting", and the casting of Fran was much more stunt casting.
Many character actors resent "stunt casting" as it takes traditional chartacter roles away from them.
CZJ at opening night party for A Little Night Music, Dec 13, 2009.