When should one correct the Imam, and should one correct him if he changes the vocalization (harakat), or adds or omits something? And what should the worshipper do if he knows that the Imam will become flustered if corrected?

If the Imam makes an error in recitation that alters the meaning, it is obligatory to correct him, whether it is in Surat Al-Fatihah or other parts of the prayer. However, if the error does not alter the meaning, it is preferable to correct him, but not obligatory.