Is repentance from reviling the religion considered repentance from mocking prayer, or does he remain in his disbelief, and how should he be dealt with?

When a disbeliever returns to Islam, all sins committed by him are forgiven, including ridiculing prayer. He is not required to perform a special repentance for that, as Allah says: "Say to those who have disbelieved, if they cease, what has passed will be forgiven them" (Qur'an 8:38). And the Prophet, peace be upon him, said: "Indeed, Islam demolishes what came before it." One must be careful not to fall into acts that necessitate apostasy, as that may lead to the wrath of Allah the Almighty.