This is a tidy puzzle, a tragic backstory, and also a complete loop for a one shot with a ticking clock. All well done!
The presentation as a Shadowdark adventure specifically is only thin because there are no typical random encounters, no certain necessity for a torch or spelunking, no treasure.... but, as a point of design if this had been explicitly system agnostic all of these things might be provided in some other context.
As is I believe that The Pale Palace feels like it would be a good finale scenario to a regional story or hook that leads players to investigate and for that it sparks the imagination on how one might get there.