Does the Imam's prayer become void if he intended to pray 'Isha then believed it was Maghrib prayer during the Salah, and does the prayer of those behind him become void?

If you begin a prayer with the intention of performing Maghrib, it will not be valid as Isha, even if the time for Isha has already begun. The determining factor is the intention. You are required to pray Isha, and the repetition of Maghrib would be a supererogatory (nafilah) prayer. The validity of a person performing a fard (obligatory) prayer following someone performing a nafilah prayer is affirmed. Hesitation in discontinuing the prayer does not invalidate it. If the Imam's prayer becomes invalid due to a reason hidden from the congregants, their prayers do not become invalid.