Were there other issues involved in the fatalities around 12-14k ft? Freezing etc?

In aviation rules you can have passengers at 12k ft without oxygen for an unlimited amount of time. The crew needs to use oxygen if you're between 10k and 13k for more than 30 minutes. Above 13k both crew and passengers must use oxygen immediately (EASA rules, FAA is different).

So they seem to consider 12k to not be dangerous to passengers.

Speaking from experience in the mountains: 12k at rest and 12k under subsequent days of exercise produce very different responses. What might be a mild headache sitting in the back of a plane could be a pretty distinct AMS case lugging a pack up and down mountains.