Is it obligatory to repeat or make up the Isha prayer in which the Imam inadvertently added two rakahs and did not sit for the final Tashahhud, and the worshiper followed him, thinking that following the Imam is obligatory even with the addition?

As long as you followed the Imam while ignorant of the prohibition, your prayer is valid. As for the Imam, if two trustworthy individuals say 'Subhan Allah' to him, he must retract [his action]; otherwise, his prayer is void. The Imam's prostration of forgetfulness before the Taslim, despite his addition, might be due to his adherence to some scholars, such as the Shafi'is.