Does the prayer of the one being led invalidate if the Imam makes a mistake in reciting Al-Fatiha? And is it permissible for someone who has combined the Maghrib and 'Isha prayers and then returns wanting to pray 'Isha to lead the people in prayer?

The prayer of a congregant behind someone who made a mistake in Surat Al-Fatihah is valid if the mistake was not intentional or if it was an obvious, teachable grammatical error. The congregant must correct the imam's mistake. However, if the mistake was intentional, or if the obvious grammatical error changes the meaning, the prayer of those behind them is invalid. It is permissible for someone who has already performed the obligatory prayer to lead the congregation.