Is the repentance of one who has insulted the Messenger (peace and blessings of Allah be upon him) and insulted Allah, and repeatedly abandoned prayer, and whose apostasy has recurred, accepted? And must he inflict the prescribed punishment upon himself?

Cursing Allah, religion, or the Prophet is an act of disbelief that expels one from the fold of Islam, by the consensus of scholars. Whoever commits such an act and then repents with sincere repentance will benefit from it in the sight of Allah. He is not obligated to go to the Imam for the execution of the prescribed punishment; rather, he must repent and conceal his sin. If what the questioner mentioned is merely whisperings (waswasah) and unreal thoughts, he will not be held accountable for them.