Tx for your thorough analysis. This is the way I have it in my "speculative" working database, deduced from your material and others. I have seen my obsession Landon McKeehan speculatively placed in the family of James and Elizabeth Lauderdale as well. Where the GreeneCo McKeehans came from is a resistant mystery. PA is the best idea since so many allied and unrelated Scotch-Irish and German (PA Dutch) families came from there into GreeneCo.
Landon's wife's family, the German-speaking Girdners (Goertners) came from NorthamptonCo, PA in 1792, but that was obviously later than Landon and other McKeehans. Where and when did the Lauderdales come in? If one could link an allied family's migration from PA with the arrival of the McKeehans, that might give a clue.
PA is loaded with McKeehans, but they seem to be clustered in CumberlandCo, PA. I have been unable to find a link or clue of linkage despite a lot of information on them. The 1928 work, The Families of Joshua Williams and John McKeehan of Cumberland, County, speculates that that groups origin was the area south of Philadelphia around Newcastle, Delaware and Maryland, which was a mecca for Scotch immigrants from Ireland, went into PA to escape stiff taxes and tithes in the early 18th century. Quite a few moved into disputed territory between Lord Baltimore and William Penn's people. Concievably, many couldn't get clear titles later and were mostly the ones that emigrated south while the luckier ones stayed on.