What should I do to atone for the sin of praying behind a tomb-worshipping imam without sincerity, intending to repeat the prayer?

Prayer behind an innovator is valid as long as his innovation is not one that constitutes disbelief (kufr). Performing it behind him is better than performing it at home. He must be advised and guided. There is no sin in repeating the prayer to dispel doubt and achieve certainty if you believe that the one you prayed behind is a disbeliever. However, do not rush to judgments before consulting scholars, as not all innovations constitute disbelief.