Is the repentance of a Muslim who secretly cursed the Prophet Muhammad accepted, or must the legal punishment be carried out against him before his repentance is accepted and he returns to Islam?

Scholars have agreed upon the disbelief and apostasy of anyone who insults the Prophet, peace and blessings be upon him. His prescribed legal punishment is death by consensus, and his repentance is accepted between him and Allah, so he is killed as a Muslim after his repentance. As for one who insults Allah the Almighty, his repentance is accepted, and he is not killed. Sheikh Ibn Taymiyyah's view is that the repentance of one who insults the Messenger, peace and blessings be upon him, is accepted, but he must be killed. After the death of the Prophet, peace and blessings be upon him, no one has the authority to waive his right, hence it is obligatory to kill one who insults him, while accepting his repentance between him and his Lord. One who has committed such an act is advised to repent and seek forgiveness and not confess his crime before the judiciary.