I assume the location Grayson mentions to be in the general vicinity of current Hanna OK just north of the Canadian. This area also fits with Capt Maxwell Phillips report of Hillabee being 75 miles from Ft Gibson.
Not much clue as to Adair's letter from "Camp Near Willaby". Adair says he's 35 miles from Spring Town -- I don't know where that is. The only other clue is it is on the route of Phillips Feb 1864 Expedition and that he sent a scout to Rabbit Ford (by present Muskogee).
As you said, the old maps don't help much 'cause places move or there are multiple places with the same name and we both know the old maps aren't necessarily correct anyway. One post-war map shows a Hillorby Square which, if I remember right, is between Deep Fork and North Canadian.
Anyway I'm hanging on to my best guess that the "Camp Near Willaby" is the same place Grayson refers to on the northside of the Canadian about 20 miles upstream of North Fork Town but, as always, I'll be watching for more clues.