What is the ruling on using a specific supplication that contains prayers upon the the Prophet and other phrases, and is someone who uses it considered an unbeliever who must perform ghusl and embrace Islam if its invalidity becomes clear?

This supplication has not been authentically reported from the Prophet, peace and blessings be upon him, nor from his Companions. It should be avoided due to the ambiguous phrases it contains. The best form of invoking blessings upon the Prophet is the Salat al-Ibrahimiyyah. Reciting a supplication with this wording is not considered disbelief and does not necessitate a ritual bath (ghusl (definition)). The nullifiers of Islam are well-known, so there is no need for obsessive doubts. One must strive to learn the rulings of Islamic law and refer to well-grounded scholars when in doubt.