Also, Arlo is out there somewhere with a gun, I wouldn't put it past him to have pulled the trigger as well.
It would be great if Arlo is putting everyone on with the dimentia act and ends up with the Quarles reward money. Probably not likely given the scene with Raylan's mother, but something just seems off about his behavior.
He's definitely not putting on an act. We know he's off his meds, and we've seen in his mind that Helen is there. For it to all be an act at this point would make the scene with Helen totally unfair audience manipulation and this show wouldn't go there.
Boyd was knocked unconscious (his head hit the handrail, didn't it?) and the single shot fired wasn't from a shotgun so we know it wasn't Johnny. However, Johnny right away was yelling to the cops that Quarles did it. The question is, did Johnny say that because Quarles now had Boyd or did Johnny say that because he needed to protect the actual shooter? Also, if Johnny was conscious and had a view of the situation how did he not kill Quarles himself?