How can one deal with an Imam and Khatib who used to glorify the former president and his party, and who incited discord in the mosque after the fall of the regime?

Praying behind an innovator (mubtadi') is valid as long as their innovation does not reach the level of disbelief (kufr). If it is an act of disbelief, then it is not permissible to pray behind them, and whoever does so, their prayer is invalidated. Although prayer behind a disobedient Muslim or an innovator is valid, it is nevertheless disliked by the consensus of the scholars.

The Imam should be advised and his errors in methodology or his appeasement of corrupt rulers should be clarified to him. If he does not respond, then assistance should be sought from those who can exert pressure on him, while exercising gentleness and wisdom to avoid stirring up discord in the mosque.