It sounds like you have two Isaac's here. There was also a Captain Isaac Williams who lived in Lawrence County Indiana (Born 1779) who was a Quaker - he is not related to my family. He came from NC also and moved to
IN from Tennessee, I believe I read. Have you seen the book "Daniel Guthrie and Allied Families of Leesville Settlement? I have the book. Isaac, who married Sarah Sally Guthrie in 1797 (which seems too early to fit your line), had a son Isaac, who was born in 1810 or 1811, and died 2/5/1872. I had him mixed up with his father. He married and had three kids but all of his estate went to his brothers and sisters. Maybe you descend from one of his missing children! Isaac Senior lived in Daviess County with his children from his second marriage to Mary flinn Evans. I have the names of the three but not in front of me (Andrew Jackson Williams, Elizabeth Williams Critchlow and one other).
I can look up whatever you want from the Daniel Guthrie book. Pat