What is the ruling on the prayer of an Imam who omitted the middle Tashahhud by mistake, then returned to it before fully standing upright, and performed the two prostrations of forgetfulness individually after the prayer?

If one forgets the first tashahhud and stands up for the third rak'ah, then if he has not yet stood upright, he must return. If he has stood upright, it is disliked for him to return. If he has begun reciting, it is forbidden for him to return. If he returns knowingly, his prayer is invalidated. In this case, if the Imam returns for the tashahhud before the congregants stand up, he has done well in performing the prostration of forgetfulness (sujud as-sahw) because it is obligatory. As for the congregants, they must follow the Imam in this prostration. Whoever intentionally abandons it, their prayer is invalidated. As for one who forgets, they are excused, but an ignorant person is not excused and must repeat the prayer.