Did you ever try to track down Emilio Drigo or his descendants? I believe he was from St. Louis? In the Gassler Manuscript, page 29:
Ruth was in contact with a Manuel Peralta (?) living in St. Louis who claimed he visited the mine in 1891, that would have been after the flood which was the cause of the death of Jacob Waltz and claimed the place was terribly broken up and almost impassible..
Another avenue to try is to track down the bulk of Adolph Ruth's research materials- most of which was left behind at his house in Washington D.C., he brought only the most essential stuff with him- directions, maps, and the rest were in his head. After he died, they ended up with his daughter, Stella Hawkins. However, as Glenn Reynolds pointed out, Stella had said that "I have a niece that's very greedy, she takes whatever she wants out of my house and it makes me so mad!" This niece as you know is Eleanor Clark. I wonder if anyone has tried to track her down, as she presumably has the bulk of Ruth's research material, including the alleged PERALTA book? She would also have all of Erwin's letters. She should still be alive.