Were JWs really prosecuted by Nazi Germany or is it another make-believe creation by the Watchtower?
Yes, they really were. Any historian can tell you that. There's one bitter ex-Dub who wrote a book about how it's not really true, and I know a lot of ex-Dubs take that as gospel because they're looking for one more thing to blame on the JWs, but he's flat-out wrong.
I encourage anyone who wants to know about what happened with/to the JWs under Nazi occupation to visit the U.S. Holocaust Memorial Museum. They do a wonderful job of explaining the persecution of JWs and have even done special mini-exhibits focusing on them from time to time. If you identify as a JW or someone with an interest in them when you first arrive, they'll also give you a little "passport" of a JW whose story you can follow on your way through the museum. I believe they also used to give out purple triangle pins if you wanted to wear one; they did the same with the symbols for other groups.
Also, a trivia bit for anyone who has gone or is planning a trip: The huge wall-sized photograph of a man in his concentration camp uniform that you see when you step off the elevators on the 4th floor (to start the tour) is a JW. I can't remember his name, but he used to speak to JW tour groups frequently. I saw him speak when I was maybe 10 or so. Incredibly moving.