How sound is the permissibility of supplicating for marriage to the Prophet Muhammad (peace and blessings be upon him) in Paradise? Is it forbidden? And what should one who has made such a supplication do? What is the evidence?

The strength of one's love for the Prophet, peace and blessings be upon him, is an evidence of the soundness of one's faith, and it depends on excellent adherence to his guidance and Sunnah. Praying for a woman to be the Prophet's wife in Paradise is not legislated, because neither the Prophet nor his female companions guided to it. It contains two prohibitions: first, assuming an unseen matter without evidence, and second, it might lead the person making such a supplication to abandon marriage, which is forbidden. Lawful supplications are better than innovated ones.