Does divorce occur if a husband says to his wife, "This time, if you leave, that's it," and then his wife leaves and returns, given that he did not intend divorce and did not explicitly utter it?

What your husband uttered is an implicit divorce (kinayah) that can be interpreted as divorce or something else. Implicit divorces do not take effect without intention. If the husband doubts his intention, then his divorce does not take effect, because divorce does not take effect with doubt. Therefore, his divorce did not take effect.