For "The truth of the matter" I can turn into McAvoy or Nicholas, but McAvoy is no where to be found. I found him earlier at an Aid quest, but now he is completely gone after searching for him way too long. I also ported back, logged out, and completely restarted the game.
McAvoy is inside the Site of Power, you have to complete the storyline to unlock the site before you can ever talk to him.
If you found someone named McAvoy at an aid quest, that was probably a coincidence of the random name generator - the whole point of the Magwytch story is that Oisin McAvoy locked himself in the Site of Power so it's the only place you see him.