What is the ruling on praying behind an Imam who does not recite Al-Fatihah well or makes mistakes in it, and what is the validity of the prayer if the worshiper behind him is unaware of this? And what is the ruling on prayer if the Imam's voice is not clear when he recites Al-Fatihah?

What you mention is an illusion stemming from whispers (waswas); for it is inconceivable that all imams would err in Al-Fatihah. You must ignore and disregard the whispers, and maintain prayer in congregation. Do not pay attention to doubts about errors in the imams' recitation, and do not repeat your prayer. Disobey Satan whenever he tries to cast doubt on the imam's recitation, and remember his enmity towards you.