How can a wife who made her husband swear to divorce her if he had intercourse with his second wife release him from this oath while preserving the marriage, especially since he had divorced her twice before?

A contingent divorce cannot be undone. If the condition upon which the husband made the divorce contingent occurs, then, according to the majority of scholars, the divorce takes effect, whether the husband intended divorce or merely a deterrent. This is the fatwa that is acted upon, contrary to the opinion of Ibn Taymiyyah. So, if your husband did what he made the divorce contingent upon, then a major irrevocable divorce (talaq bainunah kubra) has occurred, and you are not lawful for him until you marry another husband and he consummates the marriage with you, and then he divorces you. Marital problems must be guarded against, and it is not permissible for a husband to leave his second wife in a state of suspension.