Is a woman obligated to fulfill a vow to fast every Thursday for the rest of her life, after she made that vow to prevent a divorce, and then the divorce actually occurred and she returned to her husband?

The matter of this vow depends on your intention. If you intended to make the fasting contingent on the divorce not occurring at that specific moment or the time it was expected, and no divorce occurred then, you are obligated to fast every Thursday indefinitely, and this obligation does not cease if divorce occurs afterwards. However, if you intended to make the fasting contingent on the divorce not occurring either immediately or in the future, then should divorce occur at any time, the obligation to fast ceases the moment it happens.