We see a lot of Tevinter-esque architecture in the physical Fade. But perhaps that's a mistake on our part, assuming that it's meant to be representative of Tevinter. What if their own architectural style, like everything else related to their culture, was simply something they stole from the elves? That would be in-keeping with the twist that their culture isn't truly their own, but a human facsimile of a much greater elven empire.
This is assuming that whatever we see in the Fade is a "reality", which it is not necessarily. The Fade is basically a "unsolid" realm that is shaped strongly by the expectations and imagination of the visitor and his state of mind at any particular point in his journey. We do know that whatever demons we find there might just be representations of the fear, anger, sorrow, sadness (, or trust, faith?) of the visitor. Their real shape (or if they even have one*) is not assured.
Moreover, when the group in DAI arrives in the Fade, there seems to be no consensus as to what it is actually like. Hawke claims it looks not like she remembers it. Also it does resemble but not repeat the DAO Fade. But not only Hawke, also Solas and Cole don't quite recognize what they see. They all claim it was "a different part" of the Fade, but I find that argument rather weak because so far (DAO - Circle, DA2 - village) the Fade had been an "overlay" to the physical world, extending and altering it, not a completely different place.
* What I mean is that we cannot even be sure that there is a physical equivalent of the entities in the Fade. They might well only exist in the minds of the visitors (like event-triggered occurrences injected in the mind at specific points, shaped as whatever works best for the individual). Any death in the Fade might not be due to physical harm but simply a death of fear or similar. But this is just a side note to extend the idea.
TLTR: whatever architecture we see there might not be there but only a representation of our (Inquisitor) expectations.
In general, your discussion is really interesting and something I had not yet thought of. We are presented a situation of mythology (all gods, elven or Tevinter) vs. the political world (Thedas). This is actually like the Roman pantheon vs. the Roman empire.
However, I can even extend this further --and, yes, now I'm getting carried away-- to the contrast between both worlds that "shared" a mythology. The Roman and Greek pantheon are basically equivalent, while their political systems have become famous for opposing world views; Greece being the "cradle of democracy", the Roman Empire representing a totalitarian system following a policy of expansion**. This might well be applied to all of Thedas, with the mythology being the least common denominator.
Then again, I am just babbling.
** Not meaning to imply reality had been so simple, but only pointing out what is most commonly associated.