What is the ruling regarding someone who catches up with the Imam in the last رکعة, and then the Imam performs the prostration of forgetfulness (sujud al-sahw) after the tasleem; does the follower stand to complete his prayer after the Imam has performed the prostration of forgetfulness and the tasleem, or before that?

If the Imam restricts himself to a single salutation to his right, then prostrates for forgetfulness (sujud al-sahw) and then performs the salutation, there is no evidence for this method, nor do we know of any reputable scholars who hold this view. Rather, the Sunnah is to perform two salutations. The prayer is valid in all cases, because, according to most scholars, only one salutation is obligatory, and the Imam has performed it. If the worshiper follows the Imam in the prostration of forgetfulness before making up what he missed, his prayer is valid according to some scholars.