Some Germans might take offense. Most probably will not as they were either not born, or not old enough to be involved.
As a writer, I'd highly recommend you sit down and decide who your audience is before beginning. The days of widely appealing novels are just about over. The #1 best seller in the US is The Bible. It didn't really worry too much about what Jews or Hindus cared about it.
As for the 3rd question, you'd have to define trouble. Legally, assuming you're from the US, you can't get in any trouble by writing short of specifically threatening something or someone else with violence. If you're looking for a critical success, then as long as it's in the context of a specific time, and from the perspective of a character, I don't see it as being damaging in any way.