After looking at the map that was posted by the family, I see that Highway 76 is definitely included. I've had a bad feeling about that road all along, the reason being because it would have much less traffic on it than the roads closer to the casino. There are a lot of roads in Arizona that lead to and through indian reservations that I have driven myself. The majority of them are pretty desolate and lightly traveled. You may drive 4 or 5 miles down the road before you pass a car coming from the opposite direction, and then you may go another 4 or 5 miles before you see another one.
Whatever happened to them, apparently it wasn't witnessed by anyone. With that in mind, the more desolate, curvy, and lightly traveled the road is, the higher likelihood that an accident could have happened without it being observed.
Of all the 3 roads shown on the map, I would think Highway 76 would be the top candidate.
Of course, the huge flaw in my theory is that Paul's phone pinged off the Deer Springs tower, and not the one at Pauma Valley, which is literally right next to the highway 76 turn-off. If that is the route they took, I would think that his phone signal would have been bouncing off that tower like a ping pong ball. To my knowledge, no phone ping was ever registered coming from the tower at Pauma Valley.