I believe that, absent some original research, we've hit the wall on the ancestors of Mary (Polly) Dunbar Farris. A number of people have been into this question for some time, and have met with little success. Nevertheless, it's always worth posing the question in hopes that a new document will be discovered.
Mary was the daughter of Jemima Charles Dunbar who is documented in the History of Nodaway and Atchison County Missouri, 1901, in the section on James Scarlett. Jemima's father was Joel Charles.
Jemima's second marriage was to Samuel Scarlett, Jr., documented in the History of Nodaway and Atchison County Missouri, and she is listed as a widow with three children, Elizabeth, Mary, and Margaret. While we know her married name, Dunbar, we have not been able to find convincing evidence of who this Mr. Dunbar was. We also have no ancestry on the Charles line beyond Joel.