Submarine anecdote: we'd bump up oxygen levels a little bit to make the crew more alert. Oxygen had a max spec that we worried about because high oxygen could cause fires.

Anecdotally a submarine crew is too exhausted to feel effects from CO2 when it's in spec. If atmospheric controls were down and CO2 shot up, headaches were normal. Far from a controlled experimental environment.

I'm going to guess no, based on the fact that in Apollo 13 they had plenty of oxygen, but they needed to stop the buildup of CO2 or they would die, no amount of oxygen would help them, since CO2 is poisonous.

That happens anytime, anywhere - we just haven't bothered to care to measure it yet.

Won’t help unless there is a lack of o2 (people passing out and suchlike)