All the conclusions you've drawn have been correct. I've tried some of the searching you propose to do and either haven't found any John Jones, or have found too many John Jones and no way to identify which one is the right one! Many of the kids who signed Roby's autograph book lived in Mud Springs, one in Colusa, Folsom and Michigan Bar, but most in Latrobe. I decend from Lucy Ann Nichols Sanders Bennett through her dau. Roby and Roby's dau. Ida.
===Update====
I went back to my big binder for the Sanders family and reread the hard to read writing and found a least a clue about John Jones. It stated that he was 31 years old at the time of his marriage to Roby in 1874, making him born abt. 1843 In the 1860 census there is a John Jones of that age from Upper Placerville. Could be the man! I will continue to look....
Thanks for inspiring me!

Patty