Is it permissible to pray behind an imam who supplicates, seeks assistance, and swears by other than Allah? Is praying in a second congregation in the mosque's antechamber valid? And did Sheikhs Ibn Baz and Ibn Uthaymeen permit that?

Praying behind someone who invokes other than Allah is not permissible if he commits an act of disbelief (kufr), and proof has been established against him, and his apostasy has been confirmed. However, if his action is not an act of disbelief, or if proof has not been established against him, then the default is that praying behind him is valid. If his Islam is presumed, then it is obligatory for the congregation to pray behind him. As for establishing a second congregation after him, it is not appropriate due to the division it causes. But if his disbelief is confirmed, and there is no overriding harm [in establishing a second congregation], then there is no objection to their action; otherwise, they should look for another mosque.