Decompression drill for my airbus is:
Alt Selector Knob. Turn left and pull
Hdg Selector Knob. Turn left and pull
That could explain the 90 degree left turn (in hdg, hence the drift) and the descent.
Oxy mask failure gives a time of useful consciousness of 1 min or less.
I`ve had flights in parts of Europe where acars has "lost signal"
The only flaw in my theory is the transponder but lets suppose the PNF selected 7800 instead of 7700 in his haste.
Just a thought....