What is the ruling on wearing the niqab, with evidence, and is it permissible to wear it if that leads to the mother abandoning the niqab after she swore to do so?

The preference for the obligation of covering the face has preceded. Accordingly, you must act upon this opinion, and it is not permissible for you to uncover your face to please your mother, because "there is no obedience in disobedience to Allah." Your mother's view that the niqab is desirable does not obligate you to follow her opinion. If she removes the niqab, there is no blame on her as long as she does not believe it is obligatory, and it is better for her to remain in it. It is recommended to be gentle with your mother and try to convince her through righteous people.