Sorry it took me so long to reply, I just saw that you had responded to one of my queries.
Yes, there is confusion with this name (this is true in many family lines besides this one). John Miles Duvall's Revolutionary Patriotic Service includes an "Oath of Allegiance" in St. Mary's County 1778. He is also listed in Maryland records after the war for providing supplies to the Revolutionary cause of which he was trying to get reimbursed.
His son, Gabriel, also had a son named John Miles Duvall and Jacob Duvall (probably after his grandfather?); thus, the confusion!
There are other possible records for this "Senior" John Miles Duvall, since it has been told he went by both of his names, John and Miles, according to his son Gabriel. (Don't know of the documentation for this since this is not my direct line.) However, there was also another John Duvall, a cousin, by that name during the same time period.
The documentation for John Miles Duvall (Sr.) being the son of Jacob Duvall is based on his place of residence during a specific time period. It is the same place and time as Jacob's place of residence; thus, they are father and son. Harry Newman Wright received this information from a John Miles descendant at the last minute before his book was published in the 1950s. Since he was a well-known genealogist, he agreed that John Miles was probably the son of Jacob. (However, I would think that Jacob and Mary Bourne also had more children, but who they are and what happened to them remains to be found!)
Another good source is the Mareen Duvall Society. They have a lot of resources, including family group sheets for many sub-lines. They can help in perhaps documenting some of your wife's direct line.
If your wife is interested in applying for DAR membership, or you would like to contact the Mareen Duvall Society, please write me privately and I will be glad to assist.