True. Not every realm should be like those exceptional cases. But some certainly could be, and probably more places fit those exceptions in BM than in real medieval Europe.
One factual critique I'd make is your reference to "monolithic national cultures." That just ain't so. A person from southern France might not even share a real common tongue with a Parisian. Latin was universal for the highly educated. Rather, there were homogenous local cultures united by a monolithic Catholic (and, by extension, European) culture. The point about homogeneity is still valid, though.
Except that in BM, you have a ton of religions bordering each other everywhere. So since "border regions" mixed faiths a lot (relatively) in the real world, and that almost all realms in BM share a border with a realm of another faith, it's hardly surprising that people are so often soft on the issue. Other than Sanguis Astroism, I don't think any one religion has ever gained a significant contiguous spread so that any realm would be surrounded by nothing but fellow faithful.
