Is one person, or two in a closed room overnight the same, or similar to a conference room with say, 15-20 people for several hours? My apologies if you addressed this.
I do think this area is understudied. It's just that being afraid of the air supply is such an easy way to nocebo yourself.
The lowest number this article talks about being bad is 945ppm. A ppm of 600 is discussed as "fairly low" co2 levels. They are clearly not boohooing over the entire range of 1-2000.