Here's how it goes:
Imagine:
Thedas - a big sphere
The Veil - a small sphere inside the Thedas sphere ( a sphere within a sphere)
Fade - smaller sphere inside Veil sphere
Pocket dimensions - even smaller spheres that are inside the Fade sphere, past the Veil and Thedas sphere.
1. You can't see the Black CIty from the pocket dimension, not because you're not in the Fade, but because there's a block that's preveting you from seeing it. Just like how you can't see the Sun from inside your house. You can't see it not because you're in a different galaxy, but because your house has a roof.
2. Some of the Eluvians lead to these pocket dimensions. Normally they way you'd go when trying to get into the Fade physically is to go inward (following the sphere picture) - tear down the Veil sphere, and then walk into the Fade.
Now Corypheus knows that one does not simply tear down the veil, so instead he planned to use an eluvian which bypasses the Veil but doesn't directly lead to the Fade. Instead you go to the unveiled pocket dimension, which are totally within the fade, or beyond the Fade sphere. But now, to get to the Fade, you have to go outward - destroy the "ancient barriers" as Morrigan puts it, and walk into the Fade physically.
Long story short, pocket dimensions are beyond the Veil and within the Fade, or I guess "beyond" the Fade sphere. Veil separates the Thedas sphere and the Fade sphere, and also cuts off elves' connection to the Fade - which means nothing elven, not even the pocket dimensions and its doors worked when the Veil basically created a blackout on the elven power supply. It's like a dampener for magic. Any form of magic against the veil is countered by it. Similarly, any magic from the Fade that tries to go into the living world is countered, so it only makes sense that the veil gets thin when you pour a sh*t load of magic on top of it.
3. Now on to the topic of where the Evanuris are imprisoned. I do not believe the they are in the Golden City. In fact I believe that the Golden City predates everything, other than the Maker or whovever or whatever created the Fade. So looking at everything, the Maker is still number one, if he exists - because he predates the ancient elves, the Old Gods, the Titans, and all the muggles (everyone else)
The elves, as powerful as they were before, relied heavily on the Fade, unlike the Golden City, which is totally constant despite the unending changes in the Fade. Meaning, Fade rules don't apply to the Golden City.
If Solas had so much power that he created something that's not even ruled by the Fade, and put it directly inside the Fade, not even in a pocket dimension, then why would he even bother creating the Veil and cause damage to ALL elven people? With that much power he could've just kicked the Evanuris' ass, or imprison them without screwing over everyone else.
This gets into a different topic, and I'm not sure if I want to talk about it or make a thread about the chronoligcal order of all things in Thedas.