What influence did Wagner’s music have on nationalism?
What innovations in compositional technique, harmony, staging and production did Richard Wagner employ?
What influence did Wagner’s music have on nationalism?
What innovations in compositional technique, harmony, staging and production did Richard Wagner employ?
This addresses the first question only, since I consider the second as subject of reference works and whole books.
The German Brockhaus from 1894, so a nearly contemporary work, offers this text:
Obwohl W. vielfach und heftig angefeindet wurde, namentlich von den Fachmusikern, so lenkte doch jetzt der glücklich beendete Deutsch-Französische Krieg von 1870 und 1871 die gehobene patriotische Stimmung ganz besonders auf ihn, weil man in seiner Behandlung alter deutscher Sagen eine neue specifisch deutsche Kunst erblickte.
My translation
While W[agner] was often and vigorously attacked, especially by professional musicans, the German-French war of 1870/71 with its lucky result directed the elevated patriotic mood especially on him, since one considered his treatment of old German legends as a new intrinsically German art.
So at least this article puts cause and effect the other way round.
Wagner revolutionized opera through his concept of the Gesamtkunstwerk ("total work of art"), by which he sought to synthesize the poetic, visual, musical and dramatic arts, with music subsidiary to drama. He pretty much codified opera into what it is today.
He single-handedly popularized the Leitmotiv, which has become inextricably linked with the name Wagner. Composition utilizing Leitmotif has since become standard not only in opera, but also in modern film scores (the most recognizable example to the lay person is the recurring themes in the "Star Wars" film franchise).
Politically, he is remembered as an outspoken anti-semite, long after his death his melodies were adopted as theme music for the Nazi party, and therefore performances of his works are still discouraged in the state of Israel today.
Read all about it here:
https://en.wikipedia.org/wiki/Richard_Wagner#Influence_and_legacy