Jack, you said that "The opera was written in 1846-8, so it predates any political expression of German nationalism", but German Nationalism started to take shape much before that. It was flourishing at the time of the Napoleonic Wars. With the end of the Holy Roman Empire, and the subsequent defeat of Napoleon there was both a feeling for some sort of unification of the German speaking peoples and the opportunity for it. German Nationalism was, in fact, a driving force in many of the Revolutions of 1848, which you can Google, just as well as I can.