This is so interesting that I had to share my theory ....
I think Lonnie left the fair at closing -- say around 11 PM; by this time, he had somehow lost track of anyone who could give him a ride to his small, out of the way hamlet. The latest he could have eaten would be around 10 PM.
Poor little guy started walking home, hoping to get a ride along the way. He's heading from Orofino to Greer on HWY 12. Just a little kid on the side of the HWY.
The Killer on the Road spots him. I believe that he was heading to Orofino, so he would get a good front view of Lonnie. But, he doesn't react immediately - just keeps driving, in the opposite direction, thinking about it.
During this time, the two teenagers pick up Lonnie and drop him on the bridge near Greer. I don't believe that Lonnie was expecting to get a ride in Greer - the teenagers said so to excuse their callous behavior. They just didn't have time or gas to take Lonnie home. The time is now 12:40 AM
Now we know that he had undigested food in his stomach, so he had eaten at most four hours before died. The time of death was put about 1 AM; we can stretch to 2 AM, I guess. So, things happened very quickly.
Poor Lonnie is just waiting there (I don't think he gets to Greer) when the killer, who has made a U-turn on HWY 12, catches up and lures him into his vehicle.
The killer sexually assaults Lonnie in his car; that's where his fingers get messed up. And then drives back to the area where the body was found (in the direction of Orofino). Blindfolds him, walks him down to the river, and slits his throat in an expert way. He wipes his knife off on the leaves. He continues back to Orofino, where he was probably headed in the first place.
I think the newspapers concealed the sexual aspects of the crime, it was common in 1951. These papers were not True Confessions. We don't even know what grotesque position means in relation to the hands, one coroner said there were no signs of binding. I think the earlier news accounts were more honest and the later ones became more sanitized.
It was someone familiar with the area, but not necessarily local. Someone expert with a knife; everything else can be explained by opportunity / expediency and the ritualized aspect of the killing.
As to the killer - the teenagers probably had an alibi, Orrin probably just ran into the body -- his activities, his trying to get police attention, etc., don't make sense if he planned this.
Walter Cunningham looks good. So do any out-of-town visitors to the fair and carnies. Because, even though there were many better places to dump Lonnie, the killer wanted to get rid of the body quickly and probably did not want to get lost in the back roads.
It was a spur of the moment thrill kill -- probably didn't have a shovel with him; always keeps his knife in the car.