Re: Oxygen - there's a world of difference between what's "enough" for the PAX and what's enough for the crew. The PAX merely has to be kept alive semi conscious without the risk of brain damage until a safe altitude has been reached where the oxygen masks will work for sure, while the crew has to be in a state in which it can carry out it's duties (is the flight deck O2 supply under pressure or flowing free?) At least that's what I'd be thinking as an engineer designing the systems.
No need for the O2 partial pressure for the PAX to be high enough to keep them alive at 40.000 ft for a sustained amount of time (i.e. under pressure) - if the plane's descending at 3000 ft/m it'll reach an altitude where the partial pressure of O2 will be high enough to ensure survival for all but the the weakest relatively fast. If the crew's incapacitated at that altitude then the flight will be doomed anyway.