What is required of the Imam and the congregants who did not follow him in the Taslim after two rak'ahs in the 'Isha prayer, where they completed their prayer and made the Taslim, while the Imam repeated the prayer by himself?

The congregants should have alerted the Imam by saying "Subhan Allah." If he did not understand, they should have spoken to him as needed.

If the Imam did not comply with their instruction, they should have completed their prayer individually or appointed one of them to lead them in completing it.

The prayer is valid for everyone after the mistake occurred. It is permissible for congregants to separate from the Imam due to an excuse, and their ignorance of the necessity to alert the Imam is considered an excuse, although re-performing the prayer is more cautious.

As for the Imam, since he re-performed his prayer, that suffices him. He should have completed the prayer if he realized the mistake and then performed the two prostrations of forgetfulness (sujood as-sahw) after the salām.