Perhaps what is most intriguing about the situation is not the supposition that an eccentric duke was living a double life as a well-to-do middle class furniture maker, but the societal and legal circumstances that made it such a scandal.
Firstly, the tradition of primogeniture in England which made the importance of tracking not just the nearest male relative but the nearest legitimate one. Apparently one can't be legitimatized later, regardless of when marriage took place, or that was what I understood from the book. And the rule of primogeniture led to some pretty bizarre instances of far-flung relations being suddenly elevated to the prestige of a title they probably never expected; quite the fairy tale in itself!
Secondly, all those similarities in 18th and 19th century novels make a lot more sense now: "New World" citizens obsesses with the idea of fortuitously inheriting titles and wealth from the "Old World"; long lost relatives that suddenly show up under the guise of servants or wards; double lives to every possible degree (Dr. Jekyl and Mr. Hyde, anyone?); mysterious benefactors, and sometimes connected to astronomical rises in fortunes for the beneficiary, neatly colliding with the phenomenon of the new money crowd. (A nice comparison to Great Expectations in-text by the way.)
Thirdly, the well-discussed societal fear of class shift. Impoverishment is one thing, but the real fear was upwards class mobility. I thought the comments on how the role of governess was once the last stop for young unmarried women of noble birth but poor finances and how poor families would invest in a clever daughter's future by educating her in the hopes that she could snag a governess position to be most interesting. The author elaborated on fine contemporary points such as that without detracting from the ongoing story of the mystery.
It is a bit unsatisfying in the end, and I can't even fault the author because she is quite up front about her own disappointment as well. I may be mistaken, but I thought the book was published at the end of last year, yet the addendum implies that the book was published, then persons contacted her via social media, which led to the extra bit at the end of the book. Still leaves a lot of questions unanswered, and there was definitely some sort of conspiracy in the interests of the established aristocracy at the time leading to the gaps of information (my imagination would like to propose a midnight burning of key papers), but... hm, I suppose as well-resolved as one can get with a century old mystery.