[throwawaylolx]

There's no need for an ecumenical council to reject every single doctrine other confessions come up with. The Orthodox believe that the Fall changed human nature, which means that every human after Adam was affected. Since Mary was also a human, she was also affected. It was the Incarnation, the Crucifixion, the Resurrection, and the Exaltation which allowed human nature to exalt itself back to regaining communion with God. So the Immaculate Conception is rejected by the mainstream Orthodox faith and there is no need to have a council about it.