What is the ruling on the prayer of someone who prayed Fajr with an imam who stood up from the final Tashahhud before invoking blessings upon the Prophet (peace and blessings be upon him), then the imam performed a prostration of forgetfulness (sajdat al-sahw), then stood up again, then sat down and performed two prostrations of forgetfulness, while the congregant only performed two prostrations of forgetfulness?

The extra prostration you performed with the Imam was not counted as one of the prostrations of forgetfulness (sujud (definition) al-sahw). Your failure to prostrate with the Imam in the third prostration resulted in your not following him, which invalidates the prayer according to some scholars. Repeating the prayer is more cautious. Two points of clarification: The congregant's statement to the Imam, "Sit down, this is Fajr prayer," to correct the prayer does not invalidate it if it was only as much as needed. It would have been better for the Imam to perform the prostrations of forgetfulness after the salutation for the additions, even though some scholars held the view that prostration before the salutation is permissible.