The Circles function as demon beacons, occasionally as in Ferelden, demon banquets. And the templars are sending out the invitations. Literally.
Fiona wasn't the only one pushing for a breakout, and for that matter, even Anders was pushed to his actions. A kicked dog can turn around and bite. Regardless, that's the past and if your back is against the wall, it's easy for others to second guess what is really your only out.
Better to have your eggs all in one basket, so when someone starts hatching, you can stop that ****.
Having mages in one space doesn't make them more likely to become abominations. A demon alone can bring more, then possess more people, mage or not. The circle didn't go to hell because there were lots of mages around. The only flaw here is the harrowing, and not really because it attracts demons, since they can kill said mage if possessed before he's a threat. It's stupid because it forces them to encounter them at all, when they might never have in the first place.
Fiona was at the head, and Anders wasn't really pushed at all. I mean, yea kirkwall was in a bad situation, but the dude flipped. It was not THAT bad, hence why before Inquisition you and I pretty much said the same thing about him. Nothing's changed, he was still an idiot, even though I get wanting to get it over with rather than dying slowly, which is how he saw it. That wasn't your only out, that was your death warrant. Nothing about that decision was necessary or smart of Fiona or anyone else that supported her, which wasn't many.