Is the prayer of the one being led valid if he caught up with the imam in the second prostration of the second rak'ah, and then followed the imam in not sitting between the two prostrations, then followed him in the prostration of forgetfulness before the one being led completed his prayer, and then prostrated for forgetfulness again before the taslim, believing that to be obligatory?

If the Imam forgets the first tashahhud and stands up, he should return to the sitting position as long as he has not fully stood upright. However, if he has fully stood upright, he should continue and perform the prostrations of forgetfulness (sujud as-sahw) before the taslim. A masbuq (one who joins the prayer late) who missed two rak'ahs should follow his Imam in the prostrations of forgetfulness before making up what he missed, and then perform the prostrations of forgetfulness again afterwards. The prayer of this masbuq is valid and he does not need to repeat it.