Putting aside your careful analysis for a moment - you are failing to consider the implications of a cabin that is only partially occupied e.g. 180 capacity, 60 pax. Having oxygen flow to every mask is jeopardising supply(not infinite), and to a lesser extent, pressure.
Each set of 3 seats (4 masks)has an individual oxygen generator producing enough oxygen for the time above 12000ft, when you need it.
Your consideration apply to bottle supply which is no longer in use on airliners.
About eardrum perforation, the most probable cause of it, would be a fast descent in the very last 5000ft .
To avoid that problem you should not have more than 500 ft/mn in the final descent to land and as there was apparently no other damage to the aircraft, they had plenty of time to land.
About making a PA with the mask on, MOL is right, it is a bit tricky and that's why you can only make your PA below 12000 ft, once you have discarded the oxygen mask.