We can't forget that Dwilight's geography differs vastly from Europe's. For that reason alone we can't apply the same population dynamics and expect to have logical results.
Europe is essentially one big peninsula. Spain and Portugal are separated from the northwest African coast by a small strait, thus allowing movement of the Moors into the Iberian peninsula. The English Channel separates the British Isles from the north coast of France. The northern tip of Denmark almost touches Scandinavia, and furthermore, the Vikings, who sailed to Greenland and Iceland, could just about swim across the North Sea to England. Oh yeah, and Turkey is quite close to Greece, something that has always been featured in Mediterranean history with the many conflicts between Greeks and Persians, Greeks and Trojans, Greeks and Turks, and so on.
Anyway, that's all not true on Dwilight. Sure you might get some more pronounced isolation on the main bodies of the east and west, like Echiur, Darfix in the west, and Flowrestown, Donghaiwei, and Springdale in the east. And those places look a lot larger and separated from any bodies of water than Europe as a whole (For them, inland pretty much means Austria, Hungary, and Switzerland. Just about every other European country borders some body of saltwater.)
But when you get closer to the central areas, especially near the connecting islands or Valkyrja, then you'd definitely see a lot of mixing. I wouldn't be surprised if the peoples of Eidulb, Golden Farrow, Libidzedd, Port Nebel, Port Raviel, Sallowtown, and Mimer were indistinguishable. They might differ from the ones who crossed via Valkyrja, and it's plausible that they would differ from the southerners. However, I believe that the movement patterns of humans based only on the current Dwilight map, suggests a much more mixed central population, with pockets of isolated groups in the edge regions.