You just may be right about the omission of King's Mt from his application for the reasons you state. These were VA Tories not British regulars, so the fighting was probably pretty personal with these over-mountain men. They obviously did not respond very well to threats.
I know William's brother Reverend Cornelius McGuire of Floyd KY was with G.R. Clark at Kaskasia and Vincennse (several published sources). Johnston (1906) also mentions Neal (Cornelius) McGuire at the Alamance.
In Low Brown's pension application the following could be interpreted to mean William (and Low) served as spies for the colonial British forces prior to the war.
"That some time between the two terms of enlistment [1774-1779] above stated, he was by the order of Colo.[William] Preston to Capt. James Moore, appointed by said Capt. Moore an Indian Spy. and that he acted as such for two years on the waters of Bluestone, Clinch, and New river, and was in company with William McGuire, David English, and Joseph Turner."
But in William's application he specifically states his spy service was after 1777 and during the war.
"There afterwards during the Revolutionary Struggle, by the order of Col. Preston, he was by Capt. Moore appointed as indian Spy and that - he acted as such on Bluestone, Gyandotte and the Tugg fork of Sandy for about thirteen years,..."
Considering these men were in their seventies, it is reasonable to question their memories of dates, places and names so long before. Of course the Pension act of 1832 did not authorize compensation for services rendered as British subjects so that would not appear in their applications. I don't doubt he could have served in the Militia as his brother did, but I haven't read of any service prior to the RW. I would be very interested in any information you have concerning this.
Thanks very much for your post. Do you descend from Mary McGuire (Lt. William McGuire's daughter) and William Wingo?