Must I repeat the prayer when praying behind an Imam who does not recite Al-Fatihah well, and is my not leading them in prayer due to my disagreement with their communal loud chanting of Dhikr after the prayer considered correct?

Many scholars incline to the view that prayer is valid behind imams who are unable to learn, not out of negligence on their part. As for someone who is lax in learning, prayer behind him is not permissible. You are not required to repeat the prayer in the first case, and there is no harm in you leading them in prayer, even if they collectively perform remembrance (dhikr) after the prayer, but you should not participate with them in what is impermissible.