Is it permissible to call upon the Prophet, peace and blessings be upon him, in supplication and to ask him to supplicate to Allah, and to hope for the supplication to be answered through him, or is that considered polytheism with Allah?

The Prophet, peace and blessings be upon him, is not to be asked for intercession for the acceptance of prayers after his death. There is no report that he hears the speech of his Ummah after his death, or that angels convey it to him, except for matters related to sending salutations upon him. In the hadith: "Indeed, Allah, the Mighty and Majestic, has angels who roam the earth, conveying to me the salutations from my Ummah." And: "There is no one who sends salutations upon me except that Allah returns my soul to me so that I may return his salutation." The Companions used to ask him for supplication during his lifetime, but after his death, they asked his uncle Al-Abbas to supplicate for them. Had it been permissible to ask him for supplication after his death, they would not have turned away from it.