I think I can add a bit, although not much about Mary.
I have found Elijah and Mary/Polly in San Saba Co, TX from 1880 through 1920. The list I have of their children (all born in Texas) is:
Charles R, born Sept 1872 Ada J, born Nov 1874 Lucy J, born Nov 1876 Walter L, born April 1878 Joseph R, born April 1882 James Harvey, born June 1885 Wiley Francis, born November 1888
I think I may have Elijah's parents as well. In the 1850 census of Cedar District, Cedar Co, MO is Ewel R and Susan Johnson, with a 3 month old Elijah S. Ewel was born in Virginia and Susan in Tennessee. That matches with what Elijah gives in later census records. I haven't yet been able to find out how Elijah ended up in Texas.