Been lurking and following along for a bit, venturing into a first post. Appreciate all of the thoughtful ideas and information. Such a terribly sad situation.
MOO, it does seem like their hike was planned (at least the first part) and, MOO, HS/accidental seems like the most plausible COD/MOD.
Like others, it's really hard to wrap my head around how they decided to hike in that heat with their dog and baby at all. I have small kids, one close in age to their baby. We play often outside, but on hot days (90s+) we keep our time outside with the baby short and move inside to the AC. It's really hard to imagine thinking of taking a baby out for any extended time in that kind of heat. Maybe like others have said about underestimating the heat/risk and overestimating their ability, there's also inexperience being first time parents and they got tragically unlucky with their judgment call. Just speculating. Those baby backpacks are also warm (for both parent and baby) and heavy.
Regarding the idea that they might have survived had they stayed at the river overnight and hiked out at dawn and how hiking back up SL when they did wasn't logical: staying at the river does seem like the safest thing in retrospect, moo. Seems their thinking could certainly been altered by impending HS and even alarm if a sense of vulnerability with heat/return hike was setting in. However, I also can totally imagine their concern, panic even, about the baby. An overnight would be one thing with the adults or adults/dog, but I can totally see if they didn't have a lot of water and didn't feel safe drinking the river water due to algae (or even if they did) and weren't prepared with warm clothes, diapers, or food for an overnight them feeling the urge to just get Out and back to their truck. I could especially see this if they were perceiving distress in the baby (either crying or lethargy) that would make them want to MOVE.
I read lots of ideas and questions about why they chose to go along the river toward SL (bc it's steeper and adds more mileage) versus retracing their steps up HC and have wondered, could they have been looking for other people, maybe hoping someone could help or go for help, and when they didn't find anyone took SL out so as not to burn more miles back to HC?
Finally, I read the link above about what it's like to die from HS. The convulsions/seizures/vomiting all makes me think of JG found deceased in a seated position. Would this position make sense with how HS affects the body?