Is the non-accountability of a Muslim for what he thinks to himself, as long as he does not act upon it or speak of it, applicable to imagining one of the Prophet's wives in an unfitting manner, given that this imagination does not settle in the heart?

These whisperings will not be held against you by Allah, nor will you be sinful because of them, as long as you detest them and are repelled by them. You must strive to repel them and endeavor as much as possible to get rid of them. You are on the right path as long as you struggle against these whisperings. So, strive to guard your thoughts and only think about what benefits you in your religion and your worldly life.