Of course, I have no idea whether Preuss is correct, but it is at least worth taking into consideration

Yes, but if that salutation/end of letter is a direct Henri IV/IVoltaire quote, I‘m going with Fritz styling himself as the hero of the Henriad as an in joke that‘s not entirely meant as a joke by him.

„Tabloid meets soap opera“ sounds about right. A certain friend of mine, reading the Fritz letters to Heinrich re: Marwitz for the first time: „That‘s why too over the top purple prose. I don’t buy it. Who writes like that? Are you sure these weren‘t forged?“