Those are all good points. I don't recall the USAF O2 mask causing suffocation with the supply switched off/failed. It is possible to move your jaw to break the seal and breathe round it. I would have thought hypoxia in that case. Of course my old F-15 kit has probably been superseded by something far more deadly.
Your point about the effects of panic through being unable to breathe is spot on. I do a lot of diving and recall an incident a couple of years back. Not pleasant and totally disproportionate to the reality of the situation. Certainly focusses the mind on a single issue. And not always the most important one!