Must the prayer be repeated – only – by the Imam who forgot the last prostration and then performed the two prostrations of forgetfulness after being alerted by the congregants, or by the congregants as well? And what should he do if the congregants have already left?

The duty of someone who forgot the last prostration is to perform it along with the Tashahhud, and then perform the two prostrations of forgetfulness (sujud al-sahw). If the forgotten prostration was from a rak'ah (unit of prayer) other than the last one, then that rak'ah is invalidated, and he should perform a replacement rak'ah, then say the salam and perform sujud al-sahw. Sujud al-sahw for an addition (in prayer) is performed after the salam, and for a deficiency (in prayer) it is performed before the salam. If a long time passes after forgetting the prostration, the prayer becomes invalid, and it becomes obligatory to repeat it, as well as the prayer of the congregants who followed him.