The Land of Nowhere

I'd mainly recommend that if you want to start with the Frederick the Great fanfic and don't mind that the story leaves his court 1/3 of the way through and never come back

As this is somewhat relevant to my interests and as your narration was delightful, I just googled a summary and chortled when I saw George Sand included Trenck and Amalie among her historical support cast. Trenck's memoirs (or "memoirs") are among the delightfully nuttiest 18th century writings, so I'm not surprised Sand used them. If you want a short summary, I'm providing one here and here a summary of the debunking by historians (the reality being nearly as nutty, though, just in a different way).