Is it permissible, Islamically, to annul the marriage contract and return the dowry to the fiancée, taking into consideration that the disagreements arose due to her family's refusal to hold an Islamic wedding ceremony and their desire to hold a mixed-gender one, in addition to their ill-treatment of the groom?

The wife's family has no right to demand the husband divorce his wife or to hold a forbidden celebration. The husband has the right to consummate the marriage after paying the dowry, and he has a greater right to her than her parents. Obedience to him is more incumbent upon her. If he is prevented from this without justification, or if something forbidden is stipulated upon him, he may refer the matter to the judge to compel the guardian to allow consummation. It is sunnah to publicize the marriage with a walimah (wedding feast). It is advisable to involve righteous people to resolve the issue. If the wife's family insists on doing something forbidden, the husband should not participate in it with them, and he can hold a private celebration of his own.