Hi Ziljoe!
I think this is plausible up to a certain point, they would seek shelter from the cold via whatever means after all.
However, I just can't see how a snow collapse could have caused that skull fracture. It wasn't a metres tall snow cave, just a space they could crawl into. So even if the snow collapsed, it wouldn't have enough space to reach a high speed.
Moreover, the best snow shelter is one that has snow at the bottom as well, which acts as an insulator. But in that case, the collapse would push them into that snow, perhaps suffocate them, but wouldn't cause that injury.
If instead they were lying directly on rocks, this raises the question of either how they dug that deep, and why, or if this was a natural cave formed by the stream, were they crawling and lying in running water? Not the best choice when looking to escape the cold.
And in this case, the "flooring" made out of branches and clothing, was for a different den? If the collapse crushed Thibo's skull surely it would also have broken those branches.