Is the congregant who delays completing the Tashahhud after the Imam's Tasleem due to obsessive doubts about the recitation still considered a follower of the Imam, such that the Imam bears the prostration of forgetfulness on his behalf if the congregant makes a mistake in the Tashahhud during this period?

The Imam's first tasleem (salutation) severs the congregant's connection to him. Thus, the congregant performs the prostration of forgetfulness for his own oversight after the Imam has concluded the prayer. It is sunnah for the one afflicted by whisperings (waswasah) to ignore them, disregard them, and pay them no attention. He should not perform the prostration of forgetfulness for what befalls him in his prayer due to these whisperings. Rather, he should promptly say the tasleem after his Imam's tasleem and not delay.