Second from left and far right, respectively:
No, the passenger oxygen system cannot be disabled from the cockpit. An intentional high altitude depressurization would make the masks irrelevant. At 45K, you need a pressure mask to stay alive. The drop down masks will not help you.
If the plane flew for a long time and turned at waypoints, then it seems unlikely that there was any serious cockpit fire. A serious cockpit fire would likely have disabled the flight computer and the autopilot.
Second from left and far right, respectively: