Is it permissible for a fiancée to stipulate to her suitor that he wear the niqab, and if he accepts and then refuses after marriage, does she have the right to seek divorce?

There is no objection to a woman stipulating in the marriage contract that her husband not prevent her from wearing the niqab. The husband is obligated to fulfill the agreed-upon conditions, as long as they do not permit something forbidden, forbid something permissible, or contradict the essence of the contract. This is based on the Almighty's saying: "O you who have believed, fulfill [all] contracts," and the Prophet's (peace be upon him) saying: "The conditions most worthy of fulfillment are those by which you make intimacy lawful." If the husband does not fulfill the condition, she is permitted to refer the matter to a judge for annulment of the marriage.