While there is lots of 'family lore' relating to this area during the French and Indian War, actual records are practically non-existent. Why? Because as far as VA was concerned, while VA was anxious to eliminate the French occupation of Fort DuQuesne (Fort Pitt), VA did not make any effort to have the area settled. VA formally claimed it by establishing the District of West Augusta's boundaries in 1774.
Yohogania County was established in 1776; extracts from its Court records have been published by Boyd Crumrine, and can be found in a couple of places on the internet if you do a web search.
There were Euro-American settlers in the 1760s, but they were considered to be illegal residents by both VA and PA, although the Gist settlement was prompted by the (Washington family and others') Ohio Land Company.
PA did not purchase the area until by treaty in 1769, when it was annexed to Bedford County. In 1772 it was set off to Westmoreland County.
What there are records of are mainly the militiary expeditions by VA and VA-British troops against the French, beginning in 1755. The troops were not drawn from the area that became "West Augusta" and later Yohogania Co.; they were drawn from present-day VA/WV East of the Alleghenies, as well as a few from MD and even Delaware.
For these events, but not for the Braddock expedition, there is rich correspondence and other material in the George Washington Papers. The Library of VA has published some in a series of expensive books that may be available in your local library. Another group of papers at the Library of Congress have been put on the internet and are searchable, within the Amreican Memory series.
What amounts to a useful Calendar of Documents of the earlier group of George Washington Papers that are in the Library of Congress has been published by Murtie June Clark, in _Colonial Soldiers of the South_, which has a lot of material on the first military expedition led to that area by George Washington.
There may be very fleeting incidental references to a few individuals living in or passing through this area before 1762, in the published _Pennsylvania Archives, Colonial Series_. Major libraries have the whole group of Series', which are indexed. Footnote.com may have this Series on its site, I am sorry I am not sure.
So in addition to the George Washington papers, do web searches for Christopher Gist, District of West Augusta and Yohogania County for some background information. There are some summaries on Wikipedia.