Hi Miles, yes, it's a lot to keep track of, but then you have many pleasant days ahead to sift and sort through. Are you keeping a digital family tree with records for each person? and notes about "maybe" for each person? And sources - where each fact came from? If so you can just look at David's record when referring to him. Just click on his name and up it pops. Then while you are writing about him, that record is right in front of you. If you do have a birth place of Montgomery County, Ohio, that you've identified, that's news to me. I don't recall any possible birth place of Montgomery County, Ohio. Census says Ohio, in one military record he stated Greene County, Ohio (you posted the record). His parents were married in Jefferson County, Ohio (you posted the record).
But anyway, the thing about David and Cyrus is that it doesn't matter if either is a mystery to you. Marion was talking about a mystery in the family of her ancestor John Chambers F Lowe (David's brother). John's children heard references to an "Uncle Scythe." They didn't know anything much more about him except that their parents had lost track of him about the time of the Civil War. So even though your family didn't lose track of your David, the other family may have. More to the point would be any family stories or photos that you have showing that David and John were in contact.
I am guessing that when John's children heard him mention their "Uncle Scythe" it was "Uncle Cy" and to them it sounded like the cutting tool.
When, if, you get around to trying to find David's brother Cyrus, this "insight" might be helpful, as you might guess Cyrus is not located near John.