I think the letter was from Joss. The first line makes me think he was referencing their kiss. Then he writes "among my men there has been not one fatality", I believe to convey that Andy is fine. The only part that doesn't really fit is the line about the "solitary gentleman". Is he referring to himself in the third person as part of the coded nature of the letter? Anyway, the tone sounds like Joss, and he did seem to have, at least in my opinion, a much deeper connection with Carolyn.
The only thing that still bothers me about the ending is why Joss would chase after Carolyn to keep her from going back to the house, yet stop once he had the key. Because the treasure really was more important to him? Or because he knew once Nora had the treasure, Carolyn's life would no longer be in danger? He does, after all, leave the treasure with Carolyn at the end.
And finally, do you think Joss ever returns to Carolyn? Is that who she sees watching the house?