Is it obligatory to impose the hadd punishment on one who insults the Messenger (peace be upon him), or is repentance and concealment sufficient for him, and does he thereby forfeit the Prophet's intercession on the Day of Resurrection?

Repentance from a sin benefits its doer in the sight of Allah Almighty. It is therefore recommended for one who has committed a sin to conceal oneself with Allah's concealment and repent sincerely between oneself and one's Lord, without going to the ruler to have the punishment (حدّ) inflicted upon them. One who repents sincerely is like one who has no sin, and thus will receive what the general body of believers receive in the hereafter, including the intercession of the Prophet (peace be upon him).