The bounty lands awarded to Colonial Virginians for service in the French and Indian War were all in what is now Kentucky, to the best of my knowledge. I've looked at several such grants and each one explicitly stated why it was issued and included the name and rank of the person that served. These tracts were awarded after 1776, during that time period when Kentucky was still a part of Virginia.
Evidence for military bounty land in Greenbrier might turn up tomorrow, perhaps, but so far I know of no such tracts. Every grant issued in Greenbrier that I have seen states why the land was granted and all were by virtue of a Treasury Warrant -- that is, someone paid cash to the state treasury for the land -- or by right of settlement. The latter indicates that the grantee had rightful claim to land in Greenbrier before it was legal to live there and before the Indian raids forced everyone back across the Alleghenies. After the war, a commission was set up to decide who had rights to what and grants were issued accordingly.
The one exception I've seen is a grant issued in conformity with an order of a Greenbrier court. There may have been others like that but they obviously have nothing to do with military service, either.
That said, if anyone knows of a specific tract in Greenbrier County that they believe was awarded for military service, I'd very much appreciate hearing about it.
The Amerindian question is a simple matter of history. The accounts we have are a conflicting jumble but there is no credible claim that the raids in what are now Greenbrier, Bath, Rockbridge, and Alleghany, counties were by local tribes. The Six Nations, responsible for the raids during the French and Indian War, were from New York and Canada. Later raids were a result of Pontiac's Rebellion, so called, and the Shawnee raiders of that time were from what is today Ohio. The Native Americans responsible for the last attack in this area, that on Ft Donnally in 1778, were also from Ohio. It's my understanding that the mountains of what is today West Virginia were a hunting ground shared by tribes from both the north and south but were by mutual agreement uninhabited.
Do you have a source for Cavendish negotiating the release of Nancy McCoy and marrying her? I've seen this claim in more than one place but I've yet to find a primary or even secondary source for it. I've also seen it claimed that he was the first clerk of Greenbrier. It turns out, though, that apparently none of the earliest records have survived. So, like the claim that he was a quarter master general during the war, it seems to be a rumor at best.