Lets just establish the fact that by "They" you mean the few puppet magisters who took direct orders from the old gods. This is precisely why the Imperial chantry clearly blames the old gods for blights, not magic. I couldn't agree more. Blaming all mages and magic for what some magisters did is exactly what Meredith did. Its simply false.
They weren't necessarily puppets. Also, I doubt the other magisters were against the Plan.
And no, i don't blame magic and mages. I blame Tevinter and the magisters. That doesn't mean that I want all magisters dead, but simply that they're probably responsible for thedas' biggest calamity.
'Dumat' didn't point a Gun at their heads. He tempted them, and They we're foolish enough to believe him when they had everything.
There you go with chantry lies again.
Corypheus was a Magister. He was one of the ones who tried to reach the 'golden city'. He became a darskpawn (quite different and more powerful and intelligent then the normal ones), who lead the darkspwan during the fire Blight.
I couldn't care less if the golden City was golden of already corrupted. It is howewer fact that some Magister become darskpawn when following the ritual that killed thousands of slaves, and one of them was present in the first blight. So it's very possible that Corypheus found Dumat and corrupted him.