No, the Chant says that the city was golden and uncorrupted untill humans corrupted it. Cory says the city was already corrupted. That's a pretty big contradiction.
As far as banishing things, probably to the same place where demons and spirits go when you kill them in the Fade. I mean, they can obviously take physical form IN the Fade so why would banishing something TO the Fade prevent it? Doesn't make much sense to me, especially since *you* can take physical form in the Fade with the anchor.
Oh, and as far as a codex entry from the library goes, that's not really saying much since people have been coming and going for who knows how long. I mean, Mr. ENCHANTING! has a bloody CAMP there.
It was Golden before they got there. The reason people say it was once Golden and then Black is not because the Ancient Magisters told them, it's because mages noticed the difference.
It still doesn't conflict with the narrative, the magisters brought Sin to Heaven, the light goes away. Cory says he saw darkness, but it probably became corrupt the instant they arrived to it.
The theory that it was corrupt beforehand has no evidence other than Cory saying it was black when he got there. Still, I guess it is nice and fun that people would come up with theories about Andraste being Mythal and whatnot, even if the game doesn't bother giving these theories.
As for Codex entries from the Shattered Library, they still differentiate the Earth from the Fade and the Crossroads was built on the "border" between the two ways of being "present" in reality.
The codex entries were memories of ancient elves, visions that the Inquisitor witnessed, not just books that people could doodle in. If those are to be thrown out then we could possibly throw out all codex entries, since memories are pretty much primary sources compared to all the other entries that are basically secondary sources.