I agree with you that they may have died from HS. The problem is exactly what you have pointed out - the couples you mention realised they were ill prepared and discontinued their plans in order to return to safety. My problem is - why would Jon and Ellen be any less diligent?I have no problem believing that they all succumbed to heatstroke. I have been an avid hiker for years and have come across quite a few people who were not prepared to hike in extreme heat.
Even an experienced hiker may occasionally underestimate a trail, conditions, or ability of each member of the group.
I hiked last weekend and met a couple with a baby in a backpack. The man had the baby, the woman was carrying a large bag of supplies. I could see a roll of paper towels and baby supplies. Both were struggling, and at the time I passed them they were discussing turning back due to the heat. Good for them, they were realizing their limitations.
I was about two miles into an uphill trail when I passed another couple who had an Australian shepherd. The dog lunged and tried to get to the water bottle I had hooked to that side of my pack. The couple apologized and sheepishly admitted that they forgot to bring water and the dog bowl. Yikes! The dog recognized the water bottle and desperately wanted some.
The couple seemed embarrassed and said they were heading back to the parking lot, pulling their dog away.
Several years ago I went with a few friends on an extremely rugged hike where you had to sign a log book with the time you began the hike, and demonstrate to the ranger that
each person had at least 128 ounces of water. You were also required to wear hiking boots. We were several hours into the hike when we passed a couple of guys who had NO water and were wearing tennis shoes. This despite prominent signs in the parking lot stating hikers must check in and meet requirements before beginning the hike. Another sign stated that 8 hikers have died on that trail.