We do know that the stove was NOT used. It was not needed. Although the tent was doubled up (two tents created into one) the need for a stove with 9 people was not needed. Just by the body heat, there was no need to set it up. 4 people per tent, and you have 9, it was pretty warm in there already. Matter of fact, I would not be surprised if a small door (like a doggie door) was created to let fresh air in because it was so warm. Canvas tents have a tendency to warm up quick being that warm air has nowhere to go.
As an aside, my father owned a canvas tent from the 1960's (big) and it warmed up quite quick. Opening a window was a MUST just to circulate the air in 30 degree weather.
As for carbon monoxide poisoning, The stove was not needed, it was found not used at all. Not even set up. Did they go crazy from their own breathing? You gotta doubt that because everyone but TWO people were inside the tent when the chaos happened.
No stove. No lack of air. Something happened that they could not control.