As far as the children of David McKee go, you have let some of the secondary sources confuse you.Go to David's will, written in 1793, at:http://www.dcn.davis.ca.us/go/getpizza/mckee/familynews4.htmlhttp://www.dcn.davis.ca.us/go/getpizza/mckee/familynews4.html David gives 10 pounds to "Mary my Daughter now Pebbles."He gives 10 shillings to "my Daughter Margaret now Margaret McKee."
See Walter Riggs, "The Early History of McKeesport" for the most accurate version.This is available at the Historic Pittsburgh Collection: http://digital.library.pitt.edu/pittsburgh/http://digital.library.pitt.edu/pittsburgh/
James Peebles (Pebbles, Peoples), who had married Mary, daughter of David McKee, took the survey warrant of Thomas McKee, who had died.But Mary was Thomas's sister, not daughter.
By my reading, Margaret, David's daughter, was identical to Margaret, wife of William McKee, of Mifflin Township, who died testate.
John McKee of McKeesport owned lots of tracts of land, as he was a speculator in land.They were scattered all over the county.Gyon McKee sold him a warrant in Plum Township in 1789; he had received it for military service, but apparently decided to move to Kentucky instead.John seems to have had lots of cash at the time.
Whether your family, in Hempfield Township (Archibald, Samuel, Robert) was related to David is impossible to say, as far as I know.
David's son, David, died in 1785, according to probate records in Washington County.I don't think he can be identified with a Captain David McKee.