Is it permissible for a woman to marry her husband's father after divorcing his son, if the husband's father had sexually assaulted her in his son's absence, and she loves her husband and wants to preserve her halal life?

The answer to the most important point, which is the father of the husband exploiting the questioner, harassing her, and committing indecency with her, is that it was not permissible for her to allow him to do so under any circumstances. If she allowed him without coercion, she has committed a grave sin and must repent. She must take the necessary measures to prevent him from doing so again.

As for the third point, which is seeking divorce, the general principle is that it is forbidden, but there is no blame in it if it is to remove harm that cannot be removed except by it, or to distance oneself from an environment where a woman cannot protect her honor. In fact, it may be obligatory in this case if it is the only way to prevent sin.