What is the Islamic ruling on marrying a Hindu woman who outwardly embraced Islam, but then it became clear that she did not embrace it wholeheartedly and still practices her religion, given the understanding between them, the difficulty of divorce, and the desire to continue trying to convince her of Islam?

Perhaps she thinks that the rituals of her religion do not contradict Islam. So, he should first invite her to abandon them. If she accepts, then that is what is desired. If she does not accept, then he should inform her that there can be no marriage between them unless she abandons them. If she desires him, that might lead her to embrace Islam. But if she refuses even with this warning, then there is no marriage between them, and he must leave her.