I don't know if this is going to help or if you have already tried this, but Thomas Sherman Mclain was one of Ann Elizabeth Mclains sons on the 1900 census. when he got married in 1916 he had a J.C. Mclain signed the marriage record with him probably for a bond. on the same page That Anne "Bette" Mclain is living in 1900 is also a Sarah Mclain who was Sarah Starnes. Mclain. she has a son Joseph C. Mclain that is of an age with Thomas to be the J.C. Mclain who signed with him. Sarah Starnes married Joseph Mclain Sr. So this suggests to me that Joseph and Thomas might be cousins. And that perhaps one of Sarah brothers if not could be the father of those kids. That Annie never married him is probably true as on all the census records up until she dies she states she was single.
I am under the impression that your last name is Starnes also. So you might be related to Sarah Starnes McLain. I know that this is not a way to prove the name of the father but his first name might be Isaac Melvin or Thomas Sherman since she named her first two sons those names. its just a thought.