What is the ruling on one who claims to have seen the Prophet (peace be upon him) while awake, and that the Prophet guaranteed him and authorized him to guarantee whomever he wished, and then attended the Mawlid with the Rightly Guided Caliphs, the Companions, and others?

This vision is among the superstitions of some deviant sects. Seeing the Prophet, peace and blessings be upon him, while awake after his death is impossible, as is seeing any other deceased person. Therefore, it is impossible for this man to have seen the Prophet, peace and blessings be upon him, or any of those mentioned.

Celebrating Mawlid (the Prophet's birthday) is an innovation, and using visions to justify the legitimacy of celebrating Mawlid or anything else is incorrect, because rulings cannot be established based on dreams.

Seeing the Prophet, peace and blessings be upon him, in a dream has conditions, among them that he must be seen in his true form, based on his saying, peace and blessings be upon him: "Whoever sees me in a dream has truly seen me, for Satan cannot impersonate me" (agreed upon). So, not everyone whom a sleeper sees in their dream, thinking it is the Messenger, is actually him, unless his true descriptions apply to him.