Is it permissible for the fiancé to pray that his deceased fiancée be his wife in Paradise, and what are the deeds he can do so that their reward may reach her?

It cannot be definitively stated that the woman will be the questioner's wife in Paradise, but there is no objection to him supplicating Allah to unite him with her. If a man loved a woman in this world and did not marry her, and he gave her dowry as charity, and he asked Allah Almighty for her to be his wife in the Hereafter, then this is hoped for him. As for the deeds that benefit the deceased woman, they are supplicating for her and giving charity on her behalf, and every good deed such as prayer, fasting, Hajj, reciting the Qur'an, and the like, benefits the deceased by Allah's permission.