Last night I was reviewing a lot of older posts and the more current info on the geography of the Harper Flat area in relationship to the vehicle and his remains. Harper Flat is roughly rectangular, about 2.5 x 1.5 miles and not flat, but with a downslope from the southwest towards the northeast, where the elevation of the south end of Harper Canyon is at about 2300 ft elevation.
There are three main ways to enter Harper Flat, and the Harper Canyon trail, where his remains were found, is not one of them.
They are:
1. Pinyon Wash Road - going south from Highway. However, this gravel, rock and sand road does not appear to be driveable by a conventional 4wd vehicle because of a series of rocks just at at the south end, precisely where the blue dot is on that great map that
@LucyOso found. The hiking trail reports note that vehicles part just north of those rocks and hike into the flat area.
2. Pinyon Mountain Road - this is accessible from the west on S2 road near Shelter Valley or from Highway 78 a bit farther north. This is the road with The Squeeze, and Heart Attack Hill, which are sections only for modified Jeep or other jacked-up vehicles such as serious off-roader groups. There are some side dirt roads that appear to bypass these areas so that it appears, to me, to be passable with care to the Harper Flat area with high-clearance 4WD and desert driving experience. However, there is a barbed wire fence with a walking passage at the end of Pinyon Mountain Wash where the Orange dot is on the map. This fence appears to be placed to prevent vehicles from entering Pinyon Mountain Wash, but it would also prevent vehicle driving east from entering Harper Flat.
3. Fish Creek Wash Road - this is a very long drive in sandy wash beds that starts off of Ocotillo Wells on Highway 78 driving south on Split Mountain Road, then to Fish Creek Wash and winds around in a long tortuous semicircle on more sandy scrub track to end up on Olla Wash and then more long sandy tracks to end up where Pinyon Mountain Road ended, on the west side of the fence. By looking at the tracks on Google, I cannot see that this route enters Harper Flat anywhere on the east side of the fence. I would guess this was 40 miles of unpaved, progressively more sandy and rocky wash road, the kind of remote travel where you take two spare tires, both on rims, to avoid getting completely stranded. (One flat is an inconvenience, two flats can be deadly - old desert driving motto)
So I think the vehicle had to be on the Orange dot on the map, not the Blue one. Only if JRF was able to drive over the rocks at the end of Pinyon Wash Road (#1) could he have ended up on the east side of the fence or anywhere else nearer the entrance to Harper Canyon. Another possibility is that he did try to cross the rocks and damaged the undercarriage, or a wheel, or suspension, or even rolled the vehicle, ending up with it disabled near the Blue dot.
If they had wanted to visit Harper Canyon, they would have parked near the north end, where there are numerous cacti and the area is called Cactus Garden. This might roughly be the area where her backpack was found ( closer to the red pointer than the purple dot, my guess). If they had been looking for archeologic sites in the rocky hills north of Harper Flat, they could have left the vehicle beside Pinyon Wash Road and walked east. If there was flooding there after his death, I think a large flood there would have pushed the vehicle north, not south into the Harper Flat area.
So right now I think they took either the 2nd or 3rd option and suffered severe enough vehicle damage that they had to hike out and the closest appeared to be Harper Canyon to Highway 78. But that's just today's thoughts.