Is it permissible for a non-Muslim to deliver the Friday sermon (khutbah), and is it valid to inform the worshippers that their imam drinks alcohol?

For the validity of the khutbah (sermon), it is a condition that the khateeb (preacher) be a Muslim. It is not valid if delivered by an original disbeliever or an apostate. Islamic jurists have explicitly stated this condition, among them Ibn Al-Wardi in Al-Bahjah Al-Wardiyyah, where he said: "A congregation of forty believers... meaning in prayer and the khutbah." Zakariya Al-Ansari objected to this, and Ibn Al-Qasim refuted him by stating that the author's words imply its conditionality for the khutbah as well. If the Imam is concealing his sin and not openly committing it, then no one should mention this to people concerning him. However, if he openly commits it, then it is permissible. Prayer behind a fasiq (sinner) is valid, according to what most scholars have preferred.