What is the ruling on completing the Tashahhud after the Imam has said the Taslim, and what should be done in such a case?

The questioner asks many questions that indicate he is afflicted with waswasah (obsessive whispering from Satan) regarding prayer. He should not busy himself with the recommended supplications and adhkar (remembrances of God) after the Imam’s tasleem (salutation of peace), but rather make tasleem with him. If anything remains of the Salat al-Ibrahimiyyah (Ibrahimic prayer), the obligatory part of it is "Allahumma salli 'ala Muhammad" (O Allah, send prayers upon Muhammad), and anything beyond that is a sunnah (recommended act).