Was their swimming pool water tested?
I can easily see the dog dying very quickly from anatoxin a poisoning, the humans, not so much BUT it's quite possible that a combination of anatoxin a with other elements may have brought about a fatal result.
Next problem is to figure out how they contracted it, if not by ingestion, inhalation or intradermal?
Was the baby exposed to the same toxins, if that is the cause of death, as the adults or did the baby pass away from heat exposure alone?
I suspect that, if we are even able to learn the details of what happened to them, there won't be just one cause but rather a cascading failure of events.
Let's say they somehow initially skirted the issue of the heat being extreme, by hiking downhill in the mid-morning hours when it was hot but not yet extreme.
They make it to the river and cool off. Briefly? For hours? We don't know yet.
Do they realize the aggressiveness (steepness, even after the gentling effects of the switchbacks) of the S-L trail? We don't know. If they did, they could have gone back the way they came down, but that would have added the mileage of the river stretch they would have had to re-cover.
So they start back up, in the now-extreme heat. (no matter what time of day they left the river, unless it was in the 4-5am range, it would be very very hot and the exertion would make it very much hotter.
OK, so they are moving along as best they can in these extreme circumstances, and now the dog becomes ill or lethargic or even collapses. Maybe from the toxin, maybe from the heat/exertion, maybe a combination. We don't know.
The family of course does not abandon their ailing dog, but carrying him along with the baby and whatever other gear they had pushes the effort required beyond the doable zone -- maybe Jon is carrying both dog and baby, maybe not, we don't know. Maybe EC takes the baby in her arms for a while, or even tries the carrier if it fits her. Every time they make any change like this they slow or stop for a few minutes, adding to their heat exposure.
Ultimately comes a sequence of events we don't know the specifics of.
We don't know if EC was 100 feet ahead because she was trying to race to the truck while Jon tried to rest or was already making no sense, whether she was "wandering" in her throes of heat stroke, whether she was putting distance due to risk of lightning etc. She didn't take the baby with her, suggesting to me either that the baby was already obviously dead (but how would they know for sure? I think that is unlikely) or that Jon was still somewhat ok, so she was leaving the baby with him while she tried to go as fast as she could (I think this more likely).
We don't know why she only made it 100 feet farther. Was she then overcome with dizziness? Had she made it farther but then came back for some reason? I suspect we will never know that answer unless evidence is found of her farther ahead, or in notes left, etc.
If the adults stopped moving as they were overcome with heat stroke, the dog and the baby would have died soon afterwards even if they were ok at the time. (Which they probably were not given their susceptibility to heat was higher than the adults. But we just don't know.)
The cooling effect of the shade from the thunderstorm cell may not have helped them much. It may have made them feel a bit of relief, as 110* degrees dropped to perhaps 100* and maybe there was even a breeze as the cell moved overhead. But the thermal mass of the ground would not have cooled much from a brief few hours of localized shade. Air has hardly any mass, so the cooling effect of air would be minimal when the mass of the ground was still so hot.
I think it's completely possible heat alone could have caused the deaths, but anything else they encountered would only have exacerbated the risk from the heat.
MOO
Interesting point. It's almost as if their deaths happened very quickly, and took them by surprise, because, as you say, if they knew in advance they were in a situation they weren't going to survive you think they'd try to leave some kind of message or markings or signs for others to receive or discover afterwards.
We don't know that they didn't, via phone at least (ie attempted text messages or notes in a note app etc). LE has not released that kind of information yet.
But I do suspect that they thought they would make it up until the moment the heat took the ability to think clearly (as has been described earlier in the thread) at which point they couldn't really make those kinds of decisions.
MOO