I do think those 02 sats are pretty much are in line with what one would expect at those heights. Those small O2 drops in saturation do not tell the whole story. There is something else going on with a much larger drop in pressure. For example, while the O2 sat might drop 10% at say 12,000 ft, the pressure drop is more like 30%.
You can die with 100% oxygen at 40,000 ft. military pilots must use pressure masks there. Pressure is what drives the "oxygen cascade" into the cells for utilization, not just O2 itself.
headache of AMS is well known to be independent of O2 sat. Plenty of people get headaches with appropriate O2sats, normal O2sats, or O2sats like others with no symptoms. The exception might be someone who is really sick like HAPE with, say, 02sat under 75 and they have multiple symptoms. Lowest O2 sat I have ever experienced up high above 20,000 was 62. With an hour of rest and 2 liters of hot tea it came up to 75. Felt far, far better there, but to put it in perspective, all of us at 75 if in the hospital would have been put in the ICU.
