What should the worshiper do if the Imam prolongs the first Tashahhud: Should he complete the second Tashahhud, or repeat the first Tashahhud, or remain silent until the Imam stands up?

It is not appropriate to add more prayers upon the Prophet (peace and blessings be upon him) in the first Tashahhud for the Imam, the one praying alone, and the congruent follower. As for the belated follower, it is recommended for him to include prayers upon the family and supplication in his first Tashahhud if the Imam is in the final Tashahhud, because the rule is that the follower may perform what is Sunnah for the Imam to perform.