Moira's excuses were pathetic (no doubt because the writers knew the affair was rubbish and they had to make the best of a bad job in trying to explain it). She didn't just sleep with him once or twice (still bad but maybe could be explained away if they were drunk or something) she slept with him for months and in her marital bed. From what I saw she didn't give a damn about Cain's feelings so to believe now she suddenly cares is hard to take seriously.
I really hope they dont reunite with Cain saving her from something. its been done before and just cos you save someone from a bad situation doesn't mean you should be with that person.