Despite the fact that the elastic on the mask was deteriorated, David still had oxygen from what I gather. When one bottle goes, dosn't all the O2, secondly, you would only need to check the aircraft and make sure each oxy mask door was open in the cabin. Wouldnt that be enough to explain if the oxy bottle had failed or not?
Afaik passenger emergancy oxy is delivered from chemical oxy generators directly attached to the masks above the passengers. You have to pull the masks down firmly to release a fireing pin which starts the chemical reaction. The larger oxy tanks are for the crew's masks as these passenger oxy gens only work for about three minutes, just enough time for the crew to decend to 10,000ft.