Is Ibn Taymiyyah's statement that the Prophet sits with God on the Throne, and that the Messenger saw God in the form of a young man, and his citing a hadith to support that, and that he was imprisoned for his fatwa on the eternality of the world's genus with God—does this constitute disbelief (kufr)?

Ibn Taymiyyah did not say that Allah seats His Prophet Muhammad, peace be upon him, on the Throne. Rather, he transmitted an account from Mujahid concerning this and the acceptance of the Salaf (pious predecessors) of it, while distinguishing between what is authentically reported from the Messenger and what is reported from others. Nor did he say that the Prophet saw his Lord in the form of a beardless youth. Instead, he cited the hadiths reported on this matter, clarifying that these were dream visions and that a person might see his Lord in a dream in different forms, and that this does not mean Allah is actually in that form. Ibn Taymiyyah also did not assert the eternity of the world; rather, he spoke of the continuous succession of events, and that their genus is ancient, and that every individual event is preceded by its own non-existence. This does not contradict that Allah is the Creator of all things.