Is saying "O Messenger of Allah" without intent considered polytheism and seeking aid from other than Allah, and is ghusl and repentance required from one who has uttered that?

Calling upon the Prophet in this manner is not considered shirk (polytheism), even if it is impermissible. Shirk is the devotion of worship to other than Allah, and you have not committed any part of that. You must cast off these whisperings (waswas) from yourself and not allow Satan to plant their seeds in your heart, for obsessive whispering in this matter can lead to great evil. So, hold fast to the truth you are upon and know that you are upon Islam. Let your heart be at peace, and do not give these whisperings any attention.