Could the jinn have appeared to the Prophet Muhammad, peace be upon him, in human form and informed him of the unseen? And could the jinn be manipulating humans to believe in Islam? And is the assumption of the existence of other gods considered polytheism?

You must cast away satanic thoughts and be certain that the Qur'an is not from devils. The eloquent Arabs were unable to imitate its eloquence, and Muslims have unanimously agreed that it is the word of Allah, revealed by Gabriel to the Prophet, peace and blessings be upon him. Cast these whisperings out of your heart, and occupy yourself with listening to and reciting the Qur'an, increasing in remembrance (dhikr), and accompanying righteous people to drive away Satan.