Are you still looking for the parents of Frank McClurg? (Since online research is getting to be so much easier these days, I imagine you may very well have already found what you were looking for. But if so, this post may help someone else.)
The Frank McClurg who was the father of Sarah Amanda Thomas's children (presumably all of them, since she chose to give them all his surname and to call herself Mandy McClurg, although I suppose DNA testing would be needed to know for sure) is already in your "Descendants of Joseph McClurg" list. He is the same person you have listed as Francis M. McClurg b. 1842, son of Alexander McClurg and Elizabeth Johnson. His name is given as Marion in most official records, but Frank is of course a common nickname for Francis.
The clincher evidence is the 1910 census, where two granddaughters, Myrtle age 5 and Virgie age 2 are listed with Marion and his legitimate wife, Minnie Corbin. Myrtle and Virgie were the daughters of Sarah Amanda's son Grover Cleveland McClurg and his first wife, Nora T. Horner (whom you do not list.)
There is a family tree on Ancestry.com submitted by Pauline Hyatt Chalfin, a granddaughter of Olive McClurg, daughter of Grover Cleveland McClurg and his third wife, Mollie Jane Pettit, that includes photographs of the half-sisters Myrtle, Virgie and Olive together. Myrtle and Virgie both lived into their eighties, and Pauline knew them as her great-aunts.
The fact that Marion and Minnie helped to raise two grandchildren from his "on the side" family would tend to indicate that his relationship with Mandy was acknowledged, and at least tolerated by Minnie.
I have no McClurg ancestry. My connection to this family is that Grover Cleveland McClurg's third wife, Mollie Jane Pettit, was my 2C2R, the daughter of James L. Pettit and Lizzie Hardin. (James L. Pettit was also born into a similar extramarital family, one of the four children of John Link and Elizabeth Pettit, who were all born while John was married to Rhoda Sexton, and raised as Pettits but acknowledged by John as his children.) You have her listed as "Mollie Manning ? Pettit". Mollie outlived three husbands; Grover Cleveland McClurg (known as "Cleve") was the first, Taylor Wilson Manning was the second, and William Leslie Wayland was the third. She also had one daughter raised as a McClurg who was born three years after Cleve died and two years before she married Taylor Manning.
There is another Pettit/McClurg connection - the "Jane ??" you have listed as the wife of Absalom McClurg was Martha Jane Pettit, born March 1854, daughter of Benjamin Pettit and Mary "Polly" Hall. Benjamin was my GGG Uncle, and a brother of the Elizabeth Pettit who was Mollie's grandmother.
This message is getting long; I will post another followup with the census readings.