Is the validity of an unmarried man's imamate in prayer a matter of dispute, and does the hadith, "There are three whose prayers do not rise above their heads," apply to him?

There is no objection to an unmarried person leading the prayer (Imamah), as marriage is not a condition for the validity of the Imamah. It is permissible for a single person to lead the Friday prayer and deliver its sermon, based on the Prophet’s (peace and blessings be upon him) saying: “The one who recites the Book of Allah best should lead the people in prayer…” If the regular Imam delegates you, no one has the right to prevent you. It is disliked (makruh) to lead the prayer if there is a non- শর'i (unlawful) reason for disliking your leadership. However, their dislike for your leadership merely because you are unmarried holds no weight. The saying “Marriage is half of the religion” is true, but it does not mean that it is disliked for a single person to lead the prayer, or that their religiosity is deficient, especially if they are unable to marry.