Is addressing the Prophet (peace be upon him) by saying, "O Messenger of Allah, may Allah bless him and grant him peace, seek forgiveness for me," when visiting his grave during Umrah, considered polytheism (shirk) with Allah the Almighty?

Seeking prayers from the Prophet, peace and blessings be upon him, after his death is an innovation (bid'ah), not polytheism (shirk). None of the Companions did it. During the time of Umar, may Allah be pleased with him, when a drought afflicted them, they sought rain through Al-Abbas and did not go to the Prophet's grave. Ibn Abd al-Hadi and Ibn Taymiyyah mentioned that this act was not narrated from any of the Muslim imams, Companions, or their successors (tabiin). It is an innovation for which Allah has sent down no authority. Sheikh Abd al-Rahman al-Barrak emphasized that the Messenger is dead and does not pray for anyone after his death.