I find this whole thread a bit confusing unless I missed something.
As I recall, whether the oxygen source is gaseous or chemical, the cabin masks will deploy either automatically above a given cabin altitude or by pilot command. When they deploy the passengers who pull on the mask will initiate the oxygen supply, whatever the aircraft external altitude. The oxygen will then be available until it runs out. The intent is to provide oxygen until a safe altitude can be reached which I think is 14000ft but maybe 10000ft.