I've spent some considerable time reviewing the terrain and the entire trail. There are several areas where they could have lost the trail and many, many trails (probably animal) that interject with the trail. Virtually every canyon leads out to civilization. The longest of these is about 4-5 miles and empties out to Indian Cove campground. 3 canyons actually lead to Indian Cove. They could very well be in that canyon and were too exhausted not to carry on. I'm sure searchers considered this and trekked all the way through. I also have to think that anywhere beyond the road is a higher elevation (with the exception of being in wash) they should have been able to see the road and cars driving. I really can't understand how they did not come back safely or have been found yet. They were to circle this big hill/pile of rocks and loop back to the trailhead.
The only conclusions I can come up with is they ended up in some slot canyon after wandering around for days and/or they are way off the grid of searches. Below I circled a southeast area of the trail where they could have ended up after wandering and trying to follow trails. The blue is possible trails they could taken and they may have been far enough back to where they couldn't see cars on the road.
Here is a zoomed in view of the same area with the blue following below a trail.This is an area of a lot of rock covers or cave type places one could hide from the heat.