We already know that the name "Henry Deletnack Debosnys" was a pseudonym, but his middle name is curious enough to warrant examination. One could imagine something like de l'Etnac, but the consonant cluster tn is especially awkward and very low-frequency in European languages.
I originally thought his full name must be an anagram, with the inconvenient left-over letters tossed into his middle name. However, now I think his middle name is a literary allusion, of sorts.
In 1870 someone named J. Cantel published a book titled Souvenirs et Impressions de Voyage en Italie under the pseudonym M. le vicomte de Letnac, where Letnac is transparently Cantel spelled backwards.
The book is written as a set of letters from "Arthur", a 15-year-old student, to his older brother, describing his travels in Italy in 1869. I can't find any other works by the same J. Cantel, or any clue about what the J. stands for.
What connection, if any, this has to the identity of Henry Debosnys is anyone's guess at this point, but it seems unlikely to be a coincidence. I'm adding Cantel to my list of names to watch for.