Asslamu alaikum,
Dear Hazifun,
As u told that 1 we assume , imam had not commited kufr then we can pray behind HIM, but u can also understand that , we have to assume also whether he had commited insult of ALLAH ALMIGHTY or HOLY PROPHET Salellahu alaihiwasallam or any other Promonent personalities esp., SAHABA KIRAM Radi allahu anhum , if HE ( imam ) commited or has such believes then we can't pray behind HIM !
Dear, will u offer prayer behind a Shia' ? no , never bec HE (Shia' ) is the great insulter of SAHABA KIRAM Radi allahu anhum ! So,like that we can't pray behind Imam whose faith or believes according to Imam ul Wahabiya Ibne Abdul Wahab , ibn e Taymiyyah , etc.,
Wassalam