I was thinking perhaps he was carried a short distance. Thinking of the difficulty of carrying a dog while carrying a baby in a backpack... how would you do it? Put the dog on top of the baby? Carry the dog in your arms? Pretty hard to hike several miles that way, even without the heat and steep climb. Of the ~7 miles they hiked, how many could they reasonably carry the dog? Alternatively, the thirsty dog simply helps himself to river water. Let me pull out Occam's Razor...
It could have been a combination too -
For example - one possible scenario - ALL hypothetical:
Oski may have walked the first stretch and then started failing. He may have then been carried another stretch to the river.
Thirsty hot dog may have drunk and jumped in the river to cool off, and may have perked up a bit.
Then they start the climb on SL, Oski walking because he has been somewhat refreshed at the river, as you say.
Then on SL, Oski may have failed again, possibly very soon into the climb.
JG may have begun to carry Oski up a part of SL.
Perhaps occasionally putting him back down, encouraging him to walk, Oski may have tried a few steps then stopped, and JG may have picked him up again and tried to go forward carrying Oski.
Maybe Ellen tried to take the baby or the dog for certain bits - maybe they alternated. Nothing is ever black and white…
This pattern could have repeated itself several times, always ascending and always with increasing temperatures.
Until clearly all perished.
All of this is just one hypothetical scenario, of which I believe many exist.
I once had a dog who I took for a long summer walk in our suburban neighborhood. I didn’t think it was particularly hot but I was a culpable, ignorant 18 year old. I didn’t know that dogs can’t sweat sufficiently to cool off.
30 mins in, the dog refused to proceed, lay down in the shade of a bush. I encouraged her to keep going and she finally did. This pattern happened 4-5 times, stopping, encouraging, restarting. Finally I saw someone coming out of their house and I asked them if I could have some water for my dog. The dog perked up with some water but the pattern continued of the dog stopping and refusing to go forward. We eventually got home ok and I learned from the experience. It was just too hot and I was an uninformed idiot who deserved to be chastised.
I hope I have learned considerably in the 30 years since!
Gratuitous photo of our current dog attached.