Even in the ghettos the Germans used intermediaries (a Judenrat) to run a lot of the day to day work. Of course for the Jews and other ethnic groups that were scheduled for extermination the level of authority was limited in quality and time. Even under military occupation western European countries had a level of local government to keep the wheels turning even if the mayor might have to answer to the local German commandant and get major policy direction from German authorities. In the book you have fours Americas - USA/CSA/RMSA/PSA, the first two under German control the latter two under Japanese control. The novel is mostly set in the PSA and there is a collaborationist government for the PSA in Sacramento which is under the control of the Japanese on a policy level, and in detail as needed. I expect the Germans would have something similar. Just like the British in India, there simply aren't enough Germans to run things on a national level let alone a local level.
After the war, you'll see a small but not negligible group of Americans who are pro-Nazi, a much larger group that does not like being conquered but is sympathetic to Nazi ideology in part or wholly, a large group that just wants to get on with life, and a small group that will fight to the bitter end including those who see no choice like Jews. With time, propaganda, and the indoctrination of youth, the hand of German control can get lighter and those fighting against the Nazis (and increasingly American forces supporting them) will markedly diminish although never vanish.
In all of their conquests in western Europe the Germans installed local compliant heads of state who had at least some local support (Pétain probably the most, Quisling possibly the least) who were the face of the new and "friendly" governments. Fritz Kuhn is a non-starter as he was a German immigrant and closely tied to the Deutsche-Amerikaner Volksbund which was disbanded when the US and Germany went to war. Lindbergh is the ideal figurehead to "unify" America - sympathetic to Germany, comfortable with the establishment/anti-semitic crowd, and an American icon who had warned against the consequences of getting involved in a European war. Who would be installed to run the CSA someone else would have to provide some ideas, and frankly am clueless about the RTMSA and PSA especially the Pacific States as the most "isolationist" figures prewar were also pretty racist which would not sit well with the Japanese, perhaps in the Japanese areas you'd see "white" governance limited to low level/local level for quite some time.
