Never ran birthright, but I wanted to thank you for keeping the setting alive.
Searching these threads, a few people in the past mentioned doing this. I was curious where people put Diamond Lake. I am thinking of just adapting the two Wind Duke cairn adventures from this AP, rather than the whole thing.
There are a few places where Diamond Lake fits, but if any of you who ran this before in Birthright happen to see this post, let me know what you did.
Given the AP wants a city-state, somewhere in the Brechtur hinterlands would fit well. Most Brecht nations are really just city-states with large territorial claims, after all. I haven't adapted it to BR myself, but I would probably put Diamond Lake somewhere in Danigau or Muden...but those are also the only nations I remember off the top of my head. If you wanted to swap the lizardfolk in Blackwall Keep for something more Cerilian, that one nation right to the side of Muden and hard up against Vosgaard seems like a good fit but would require finagling the plot a bit since the Vos aren't quite tribal in the same way the lizardfolk are. Or you could use Cerilian goblins, which would fit better.
But it's been a good decade since I looked at the setting so I could be pretty far off.
...or I could read the OP and give the advice asked for. :) The cairns would make good ancient elf things, which could put them almost anywhere. The run-down mining village argues for Brechtur still, or possible some fringe of the Gorgon's domain.