I'm thinking specifically of movies like The Great Escape or Patton, made in the decades after the war. Some of the actors were in the Wehrmacht according to their wiki bios. Others I haven't been able to find much detail on, but would have been old enough that conscription would have been hard to avoid.
Were the pasts of these actors the topic of much discussion? Did they provide insights into the inner workings/day-to-day life of the German/Axis forces? Were people reticent to cast actual former soldiers as Nazi commandants, and were they reticent to take up those villainous roles?
As you've alluded to, the lack of available sources, biographical or otherwise, on the lives of German actors working in the United States (and Western Europe) in the post-war years poses a problem in satisfyingly answering this question to the fullest. I personally am not a film scholar, and unfortunately I have not found a comprehensive study of German actors in classic WWII films working in Hollywood or the other Allied countries - perhaps such a study does not yet exist. A complete answer to this question might require an in-depth study of primary sources ranging from interviews and memoires of casting directors, actors, and studio executives of the time. Without these, a realistic portrait of the personal, individual attitudes of film industry workers toward former German soldiers in these roles is difficult to ascertain. However, I do believe I may be able to provide more clarity on the general history of German-speaking actors portraying German soldiers and characters in WWII films, and hopefully will be able to address some of your related questions through specific examples.
To begin, it is evident that a large number of major German-character roles were not going to actors who had previously served in the German military during WWII - at least when considering American cinema from 1940-1970 (the latter date being the year "Patton" was released). Two of the most well-known German-American actors of this period, Peter Lorre and Conrad Veidt, immigrated to Hollywood before the war. Veidt, who portrayed the famously sinister Major Strasse in Casablanca (1942), defiantly declared his race as being "Jewish" in mandatory Nazi paperwork in the 1930s despite being a Lutheran - Veidt's wife was Jewish, he loathed the Nazi regime, and seemingly did this as an act of solidarity before his departure to the United States. Lorre was himself Jewish, and while not primarily known for playing military characters, he built his career on his portrayal of unsettling foreign (i.e. German) characters and stereotypes.
These two examples of a pre-war flight of artists from Germany are not exceptional. Lorre and Veidt's costar in Casablanca, Curt Bois, was also a Jewish German who had fled the country, and many of the smaller German character roles in the film are supposedly played by Jewish Émigrés. German director Fritz Lang (who directed 1927's Metropolis - a favorite of Hitler's) similarly moved to the United States in the 1930s - though the reasons behind his move are less obviously principled (See: Thomson, The Big Screen, pp. 64-65, cited below). These are only a few of many examples.
This leads me to the topic of the CBS sitcom "Hogan's Heroes," an analysis of which may provide a roundabout answer to your questions: "Were the pasts of these actors the topic of much discussion? Did they provide insights into the inner workings/day-to-day life of the German/Axis forces? Were people reticent to cast actual former soldiers as Nazi commandants, and were they reticent to take up those villainous roles?" For the uninitiated, Hogan's Heroes was a half hour sitcom which ran for six seasons in the 1960s and early 1970s which involved the sabotage and shenanigans of Allied POWs in a German luftstalag (prisoner of war camp for captured enemy airmen, operated by the German air force). Some may balk at the premise today - a rag tag group of international good guys sabotaging the Nazi war effort under the noses of bumbling Kommandants and Gestapo characters, but the three main German characters in the show were portrayed by Jewish-German actors who had fled Europe to escape the Nazi regime: Werner Klemperer, John Banner, and Howard Caine (né Cohen). Additionally, the French character on the show was played by actor Robert Clary, who survived a Nazi Concentration camp in his childhood. There are quite a few interviews with these actors that you can find, and which you may find interesting if you'd like to delve deeper into this topic (I'll link a few below).
What these interviews do tell us is that yes, German-American actors of this period definitely did discuss their pasts, their experiences of the war and the horrors perpetrated by the Nazi regime while on set, and in particular while working to portray German military characters. Obviously, this is an example Jewish actors, who doubtless felt differently when it came to discussing their experience during the war, but we can at least draw the conclusion that this topic was not assiduously avoided on-set. According to Clary, he, Klemperer, and Banner remained lifelong friends and bonded over their collective experiences and those of their families.
Secondly, these interview tell us that the Jewish actors on this show, at least, had no qualms about portraying Nazis - they were not reluctant to take up villainous roles. From Klemperer: "... my background is... a totally irrelevant thing... My Job s to be an actor, and as long as it has, you know, decent tastes, I shall portray anything no matter how ugly it is or how pretty it is." Some of these actors found catharsis in skewering Nazi characters in their portrayal, as Mel Brooks has also famously spoken about.
I'll briefly address a final aspect of your question - were people uneasy or at least talking about the casting of former German soldiers in WWII military roles? While this may have been a topic for discussion or concern amongst individuals working in the industry, and I'd love to find more evidence of that, I personally am doubtful that the massive, impersonal studio system machine was significantly affected by or considering of these concerns. The ruthless business practices of Hollywood during this period are another topic altogether, but at the very least I can provide one piece of evidence to back up my assumption. Scholarship in more recent years has revealed that American studios in the pre-war years worked with Nazi censors and even acquiesced, directly or indirectly, to assisting the Nazi propaganda effort - at least until the outbreak of war. The motive here was maintaining access to the German film market - profit over principle. However, that is just my view, and it probably warrants more research.
Due to lack of sources (and my ignorance of the German language), I've focused on actors who did not serve in the German military during the war and who were decidedly anti-Nazi. However, I hope my answer gives you a bit more context for the general makeup of German-American actors in these types of roles who had a living memory of the war and goes some way to answering your related questions.
As an addendum, what I'd really like to read is an account of a Jewish-German actor working alongside a former Wermacht soldier like Hannes Messemer (The Great Escape) while both playing Nazi characters. (Not a perfect example, but Messemer acted alongside the Jewish Kirk Douglas in "Is Paris Burning?").
Sources: