I am not a researcher of these Stillwell lines but I do have information from Loudoun County.
From the Index to the 1758-1781 Loudoun County Virginia Tithables by the Sparacios:
Stillwell, Samuel - 1 tithable, List given in to John Miclehenny for 1761 Stillwell, Samuel - 1 tithable, John Mucklebary's List of Tiths: 1762 Stilwill, Samuel; Shacklet, John - 2 tithables, Tithables 1765 Stillwell, Samuel - 1 tithable, List of tithables taken by John McIlheney Gent. for 1768
You stated: All my other data is from Loudoun Co, VA, . . . . Which at the time included a lot of area as the other western counties weren't yet formed. The dates of their formations may or may not be of any help.
Loudoun County was created in 1757 from Fairfax County, which lies to its east.The borders of Loudoun County have not changed since that year, except for a very slight correction on its border with Fairfax County.Loudoun never contained lands of any other western counties.
If you have questions concerning Loudoun County Virginia records, please email me directly.