How did the blood mages in the tower kill more mages then the Right of Annulment, the Right of Annulment would of murdered every Man, Woman and Child in the Tower with no exception.
Gregoir did think the Tower was a lost cause because he called for the right of Annulment. He said as much to the Warden and told him not to enter the Tower because it was pointless.
Gregoir waits because he believes he needs reinforcements to enact the right of Annulment, a case that was proven wrong.
When you encounter Gregoir and he is overcome with emotion about their being no hope for survivors he did has not even checked to see if there are any, he even ignores the few in the next room that are preventing the Demons from escaping and the children that have to fight a Rage Demon on their own.
Gregoir accepts the Wardens word that Irvin was not possessed and the circle is safe. He had no choice to accept the word of the Warden who had just gone through the tower and defeated the Demons and saved the mages with less of a force then what Gregoir has available to him.
Most of the mages in the Circle at Ferelden were already dead (or abominations, which are as good as dead). The blood mages killed them by luring in demons.
Gregoir is *afraid* (and rightly so). He's not wrong so much as he doesn't realize he has a 4-hero crazy-good party here that can actually clean the tower out in front of him; he probably needed reinforcements to deal with the situation. Without the Warden, he absolutely would have needed reinforcements to annul the Circle and he likely could not have stopped to chat and figure out who was or wasn't influenced by a demon. That's sad, but the annulment in Ferelden would have been a reasonable decision, considering the circumstances. I'm happy there was another option, but you can't exactly loose demons upon the world.
Gregoir accepts the Warden's word but also - perhaps moreso - Irving's. Had Irving been there to tell him he was not possessed and neither were Wynne and the children, he may have let them out - he in fact says he will accept only the First Enchanter's word. This is because he knows the First Enchanter well and thinks he can assess whether he is a demon/abomination or not. He doesn't necessarily KNOW the other mages and thus doesn't know if they're controlled by blood magic or demons or whatnot.
Gregoir is a good Templar. He is moderate, he likes the First Enchanter and works with him, and he doesn't even seek to punish the Circle for what the blood mages wrought. He seems genuinely saddened by what has happened and angry as much at himself for not catching it and stopping the needless death as anything. Not all Templars are Gregoir, and that is the problem, but painting Gregoir with the same brush as the abusive Templars because he had to make legitimately tough decisions is silly.
Well, Corypheus is not in any way, shape, or form a demon. Also, because demonic possession makes you either glow or catch fire, something we've seen literally every time we've seen possession in action.
In lore, it is not suggested all abominations glow or catch fire. It is suggested they can live insidiously inside of you. In the game, this is represented by things like being able to let the demon inside Connor remain, dormant, if you choose.