Supplemental oxygen

Agreed.....what is the cost of an O2 refill vs. a bad outcome?!?!??

Poor physical health, smoking (my bad....don't harp....I get enough already....), lack of sleep, different start point (coastal pilot suddenly flying at Denver or similar....) etc.
In general, I see no real downside to excess O2 other than sorta hyperventilating.....not great, but the opposite is way worse.
When you go O2 low, first thing to go is higher thinking.....exactly what you need then.....

No....have not done an altitude chamber....
No....have not done much flying above 10K'ASL.....
Yes, did a gold climb with no external O2 to about 13.5K' on the eastern seaboard.