wrote:

Any thoughts or corrections to my reasoning?

The ATP is correct. While people do vary greatly, and some people have been able
to perform adequately without oxygen at 20,000' or more, most people can't go
much higher than 10,000' without suffering some ill effects. Because of this,
the FAA requires that a pilot use oxygen if they spend over 30 minutes above
12,500'.

In general, people can use a cranula or similar device to provide oxygen up to
about 20,000' (the FAA limits use of these to 18,000'). These simply bleed
oxygen into the air you breathe. Above that, you need a low-pressure mask. These
ensure that all you are breathing is oxygen and are good up to about 25,000'.
Above that, you need a pressure mask. Those increase the pressure of the oxygen
and work well up to about 35,000'. Above that, you need a pressure suit or a
pressurized aircraft.

The emergency drop-down masks for airline passengers are low-pressure. They
won't keep you conscious at 35,000', but they may keep you alive.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.