What is the ruling on an Imam who mispronounces Al-Fatiha, uttering the letter Qaf in a colloquial dialect, and is not convinced by advice, and is it permissible to abandon congregational prayer and pray at home in this situation?

If the Imam replaces the letter Qaf with another letter, then prayer behind him is not valid. The mosque congregation must be alerted to this. Likewise, your prayer behind him is not valid, so you must pray in another mosque if one is available. Otherwise, pray at home in congregation with your family.