Very good question.
I would've imagined LE to have announced by now something like this, after their initial autopsies AND after poisonous gas, homicide, suicide and so on were all ruled out.
COD
Dog - overheat
Baby - overheat
JG - still unknown
EC - still unknown
MOO
RSBM
While I don't expect LE to release COD for any family member in piecemeal fashion, I do think part of the seemingly protracted investigation is because COD will likely end up varying by family member as you suggest. So it is complex. I also suspect, IMO, that Oski and Miju died from heatstroke as dependents of adults who died from something more complicated... some combination of things that could include legal drugs, poison such as a pesticide or Ethelene Glycol, and possibly heatstroke as well.
And given the 9/30 update from the SO,
I think foul play could explain why they were on the SL trail, a place where few of us, if any, would go that day, especially with a baby and long haired dog with unprotected (?) paws. So let me dabble with foul play a little since Admin has entrusted us with guided permission.
IMO: What if JG and EC intended to meet someone at the trail head? or had planned a rendezvous with someone nearby that morning and were eventually taken to the trailhead? What if the meet-up was a lure to get them out of their house by a nefarious character they knew or didn't know? That meet-up goes terribly wrong and then at gun point they are forced 1.5 miles down the SL trail. There, the adults are incapacitated by the third party (pesticide, Fentanyl). JG and EC are both left unconscious (presumed dead), dog tied to JG to keep him there, baby in carrier next to JG. JG dies from the acute poisoning and the dog and babe die of heatstroke... EC comes to, in a very weakened and sickened state, sees her family is all deceased, and with horrific grief and panic, she crawls 30m towards the truck and then succumbs to heat / poison.
Of course the ability to better understand the tracking of paw and foot prints (JG, EC, Oski, searchers, LE, and then possibly nefarious entities) is key to this scenario. But I keep going back to the question, why the scene was treated as a HazMat situation - what could LE see, hear or smell that informed them to take extreme precautions?