This accident is very similar to the Payne Stewart crash. The Citation has a max altitude operating altitude of 43,000 feet with one pressurization system operating and 45,000 feet with both pressurization systems operating.
When instructing on use of O2 masks there is a lot of understanding. When at those altitudes the oxygen regulator is set at on and 100%. No O2 flowing. Mask must be Quickly to put on rapidly When in this setting pilot sucks in oxygen and breaths out CO2. However, at higher altitudes after putting a mask the regulator must be set to on. That way the pilot has to accept the flow into his lungs and forcibly push out stale air in lungs.
Guess how many pilots forget that and then think O2 is not available? The FAA at last checking requires that one Commercial airline part 121 pilot wear mask at altitudes above 42,000. There is no requirement for part 135 small plane charter ( under 12,500 # ), general aviation, or small plane pilots to do same. A single pilot airplane is even worse as you can imagine.
Depending on the auto pilot and GPS model if all way points entered with first on origination airport or first waypoint then when reaching destination GPS rolls over the first waypoint which would be either airport or first waypoint enroute.
Yeah, it doesn’t make sense for a pressurization issue.
The Citation has a bleed air system that is fool proof and unless it was shut off it would not lose pressure. It also has a horse collar that would drop over the pilots head in case of an issue to supply oxygen. Like I posted, you can follow the route it took on flight tracker. From where it crashed it looks like it had the minimum, but legal, amount of fuel on board for a flight of that distance.