Is it permissible for the regular Imam to intend a supererogatory prayer during the 'Isha prayer when combining the Maghrib and 'Isha prayers, and if his intention is invalid, does only his prayer become void, or do his prayer and the prayers of the congregants become void?

If the Imam changes his intention from the obligatory Maghrib prayer to the obligatory 'Isha prayer, and then to a specific supererogatory prayer (like a regular Sunnah prayer), his prayer becomes invalid, both as an obligatory and a supererogatory prayer. As for the congregants, their prayer remains valid because they are unaware of the reason. If he changes his intention to a general supererogatory prayer, the obligatory prayer becomes invalid, but the supererogatory prayer remains valid. Their prayer is also valid because the prayer of one performing an obligatory prayer behind one performing a supererogatory prayer is permissible.