Is it obligatory to repeat prayers performed behind an imam who clearly mispronounces Al-Fatiha, and what about prayers performed behind another imam who mispronounces other words, and is it obligatory to inform the worshipers of that?

The melodic error (lahn) that changes one letter for another and invalidates the word's meaning, as in the act of the first imam, invalidates the prayer and renders it impermissible to follow him. The prayers performed behind him must be repeated, and their number estimated if unknown.

As for the other imam, breaking the baa’ in "na'budu" is a melodic error that does not invalidate the prayer, and replacing the saad with a seen in "al-sirat" is a correct recitation. The matter should be reported to the responsible authorities to change imams who make melodic errors in their recitation.