1 – If the imaam recites al-Faatihah incorrectly and alters the meaning, his leading the prayers is not valid. Otherwise his prayer is valid, so long as he does not do anything wrong in his recitation such that he distorts the Qur’aan, or adds or takes away anything deliberately. In that case his prayer is invalid.
2 – The prayer of those who were led by him is valid, in sha Allaah, so long as they did not know that his prayer was invalid.
3 – The prayer of those who accused him then left him, if they thought that his prayer was invalid then their prayer is invalid too, otherwise it is not.