I would just try to draw her out and get her to express her feelings on the matter so that they won't be so quickly forgotten. Especially if you find that you have trouble restraining yourself when the wife is criticising the cult (i can fully relate to that). Practice restraint until you're more able to walk the line effectively.
Focus on how asking about her feelings and her thoughts. Don't give her multiple choice or either-or type questions because it will either end the conversation when alarm bells go off, or she'll be backed into a corner and forced to toe the party line. If you force her to verbally back the cult, it will cause CD and force her to change her beliefs to be more in line with the cult.
If you can get her to create a lasting memory about how she feels about this one event, that will keep getting called to mind when she sees this guy in the congregation and when he gives a talk. After he gives a talk or makes some minor (or hopefully major) error, you might say something like "you know I didn't really have much of a problem with it when he was orrigionally appointed - but you were right. I don't think he should have been appointed." This will remind her of her original oppinion on the matter (she will likely try to convice herself that she never thought that as time goes on) and force her to try and reconcile things all over again.
Of course, take this with a grain of salt - I haven't had just a ton of luck with my wife yet. It's still early though, or so I tell myself.