Yes, I think you are going in the right direction. However, the only written documentation is second hand, based on oral traditions that were confused at best.And many people have continued to confuse Zachariah Sr (b. c1722), Jr, III, etc., as well as so it has all become quite a tangle.However, since Zachariah Isbell Sr. (born about 1722) married Elizabeth Miller, the daughter of John Miller (who later moved from Virginia to Kentucky), I don't think anyone could argue against your John Miller Isbell being a grandson of Zachariah Isbell and Elizbeth Miller.Their marriage is proven in the Miller probate records in Kentucky.
Some of the confusion comes in with those who show Zachariah Isbell Jr. (born about 1750) as marrying a Polly Miller, based on tradition.(Zachariah Jr. married Catherine, according to known records.)The Polly Isbell, widow, who was in Alabama later near the Levi Isbell family was the widow of Jason Isbell.Apparently some early researchers thought she was Zachariah Jr's widow.
Zachariah Isbell Jr. lived in Sevier Co., TN and died there on land where (or near where) Levi Isbell (b. 1770), John Miller Isbell (b. 1777) and William (aka William Zachariah Isbell) later lived.Levi and William moved to Alabama. William and wife Sarah were my line, through his son Levi b. 1797 and through his son Elijah Miller Isbell, whose wife said he was a grandson of Zachariah.
I think we need to look for more records in and near Sevier Co., TN.A deceased researcher stated that these three brothers lived on Zachariah Jr's land and apparently inherited it, but I don't have those land records.I have seen reference to a tax list listing these three Isbells, however.
Zachariah Sr. (b. c1722) was the son of Henry Isbell and Miss Cox, by most accounts, and circumstantial evidence indicates theu were his parents as they were all in Orange Co., VA. and probably parents of James b.1720, Henry Jr, and William b.c1724-6 (m. Ann Dillard).Some also list Benjamin and Jason and Christopher. However, different versions (which I don't believe are correct) have said Zachariah was the son of a Zachariah Isbell b. about 1700, son of William (however, I don't think there was a Zachariah born before c1722).Someone else invented a compromise version showingZachariah (born 1722) as a son of William Isbell JR., brother of Henry.I really don't think that is correct either.