I think Jonas is dead because the berries that he ate could have been poisons and the raw fish that he ate might have affected him in some way because the raw fish was not clean or cooked it was raw. Those are all the ways he could have died by catching a disease by eating stuff.

Some other ways he could have died are hypothermia. The only thing he was wearing were a tunic, pants, and a shirt. He was not prepared for such extreme weather like snow so he could have gotten hyperthermia.

And another way he could have died is from too much pain. Because he had a scraped knee, a twisted ankle, and not enough sleep. All of this combined could lead to fatal consequences like the scrapped knee could cause some sort of diseases and the twisted ankle make it impossible to travel and not enough sleep can cause you to faint and travel less because he is to tired. And those are all the reasons he could have died.

Now it’s time for some common sense: what are the chances of him climbing a hill and finding a sled in the exact same place as in the memory? and wouldn’t the sled be buried because of the snow? What are the chances of that? I think he is hallucinating the whole thing.