Does the imamate of an imam who committed adultery and repented become acceptable, what is the ruling on our prayer behind him, and does Allah accept the repentance of the adulterer?

Fornication is a grave sin, and it is not permissible to accuse anyone of it without clear proof, otherwise, the punishment mentioned in the Qur’an will apply. The prayer behind the mentioned Imam is valid even if he committed fornication, because the principle is that whoever’s prayer is valid for himself, his Imamate is valid for others. It is preferable for the most pious and God-fearing among the people to lead them in prayer, provided he meets the conditions of Imamate.