This is one of those side tangents where you know very well that the two sides are never going to agree, yet they keep going :)

Without an actual implementation it's largely a guessing game.

To answer the nightmare scenario, in theory you could use two concurrent models and use whichever makes the task at hand easier.

I forked a sidebar with links to an actual implementation..

Maybe it's redundant, this is a long thread after all, but it seems like the room within a room is useful for when you have two paradigms going. Where you want, say, the outside space (city street) and the inside space (e.g. a shop) to line up on a grid so you can handle something like a passwall spell which would need to align correctly. After all if I there's a house with rooms and I am trying to get through the first floor wall I had better end up on the logical other side of the wall.

It could possibly be handled solely by calculation and some kind of association table rather than with model of some kind (where room1's wall lines with a particular spot on the outside), but a model seems either to get your head around. If I don't have a way to know where the outside wall of a building is and be roughly near it then that kind of spell is pointless. Which is sad, and uncool.