Must the fiancé—who has become a legal husband—be obeyed regarding wearing the niqab before marriage, or must the mother, who rejects that, be obeyed, bearing in mind that the niqab is only worn after marriage and in the marital home?

A woman who has entered into a marriage contract (but not yet consummated the marriage) is not obligated to obey her husband until after consummation. The obedience to parents does not conflict with the husband's rights. The covering of the face is a matter of scholarly disagreement among jurists. If you believe it is obligatory, then it is not permissible to obey your mother in uncovering it, and you should deal with your mother tactfully. However, if you believe it is merely recommended, then there is no harm in obeying your mother.