I'm just watching it now, but do you guys think Cullen was required to strict the mage warden down if they failed the harrowing because of his crush?
EDIT: err I just realised that probably not because he is chosen regardless if male or female
No, I don't think his crush on f!Mage mattered. Greagoir is actually awfully oblivious to the love gossip, so I wouldn't be surprised if he had no idea 
But there is still something I broke my head thinking about. Our mage baby is probably the most talented apprentice, tutored by Irving himself, so nobody is surprised when we pass the Harrowing. Irving apparently was even making arrangements with Duncan about sending us to the war even before we were tested.
Meanwhile there is this guy, Cullen. He's probably older that Alistair, because he's a full knight already. But Alistair said he was taken to watch the Harrowing during his training, and it went bad. So I guess Cullen also saw some Harrowings before he "graduated", but the mages were lucky. So Greagir picks Cullen, who has never slain an apprentice, to kill the Amell/Surana if they fail. But nobody expects that they will fail. There is always a chance, but it's a very small chance with someone so talented. So the point is obviously not to show Cullen an abomination and make him kill it. I suspect the point is to make him realize that he can't think of mages the way he does, because he, as a templar, has this power over them, and he can't keep thinking that they can be friends. Cullen, even when you play as a male mage, is very mage-sympathetic. And he makes no secret of it, so I bet Greagoir know that, and he's trying to open Cullen's eyes.
Theory number 2: Cullen brings luck. Whenever a mage is harrowed, and Cullen is in the same room, the harrowing is successful. It totally looks like Cullen has seen more than one Harrowing, because he says that ours was the best one he's ever seen. So, Irving knows that, and he asks Greagoir to bring Cullen to the Harrowing of his favourite student, because Cullen is like a lucky charm, lol 