It's hard for me to visualize a world where lines of communication were as narrow as back then. The point about group identity is great; what if the Serb village over here in the north had no or very vague idea about all these Serb villages in the south; or had no idea how many total Serb villages there were altogether? But once publishing and communication took off, perhaps people became more aware of the extent of the group and they coagulated. Germany is also a great example, starting from dozens of little states.
I can see that in urban environments and in the country cathedral and university towns also could provide information outlets. I know too little about German history to pretend to informed commentary :) but my sense of it is that the emergence of the German nation was a project that had multiple dimensions of support in society, including the nobility and the clergy. It also happened post Napoleon (occupation in parts) and that definitely may have helped gel latent ethnic affinity between the various German principalities.
But I return back to my earlier comment about songs and proles. An expert in 17th century folk songs of European people can shed light on what preoccupied the minds of the lower classes. I sincerely doubt it was ethnic fervor.
