What is the ruling on the prayer of a latecomer who joined the Imam in the second rak'ah, and another who joined in the last tashahhud, then subsequent worshippers joined the second latecomer, then the Imam remembered that he had forgotten a rak'ah?

Those who join the prayer late and catch the second rak'ah and the last tashahhud must return and follow the Imam to complete his prayer. What they did after the Imam's salutation is not counted. It is not permissible for other latecomers to follow the latecomer who caught the tashahhud, because he followed a congregant whose Imam's prayer was not yet complete. It is not valid to follow him, even according to the Shafi'i school, if it becomes clear that the Imam made salutation deficiently and returned to rectify it.