Is it permissible to complete a marriage to a fiancé whose mother refuses the marriage and threatens to disown him, and is that considered an act of disobedience?

Obedience to parents is obligatory in matters that do not involve disobedience to Allah and are within what is good and permissible. It is not permissible to defy parents regarding marriage to a specific woman unless the son fears falling into fitna (temptation/strife) if he does not marry her, or if their refusal is out of stubbornness. However, if the mother prevents her son from marrying altogether, or wishes to force him to marry someone he does not desire, then there is no obedience to her in that. It is advised to show forgiveness and pardon, not to assist the suitor in being disobedient to his mother, and to involve righteous people for reconciliation, along with supplication and seeking Allah's help.