Needs to be something that comes from the books to hold value in this search. Sadly. I have all sorts of ideas but if they don't apply to the story than I try to shy away. The rabbit trails are already everywhere with the story alone.
For solid evidence, you're absolutely right but I don't think there is one solid enough.
My way is possibly slightly unfair, because whoever created the mystery was a human, with all human thinking patterns and I try to exploit that. I'm brute-force hacking the design by making connection based on what the creator could think (eliminating false threads and finding the optimal template) when he made the secret, rather than following clues he left. Assuming that someone did it with passion (given the circumstances I think it is not wrong) he had to use existing connections even if they lie outside of the game world to give significance to the matter. And if one person could think that, all you now need is some empathy to let his thoughts guide you