Just a couple of points:-
Boeing 747-400 Flight Crew Training Manual has an illustration of 'altitude versus time' for rapid descent, and interpreting it the descent from FL290 to FL100 takes just over 2 minutes ( the high speed, no structural damage case and the lower speed, gear-down option, apparently little different).
The manufacturer offers the option in the Pax oxygen system of either supply oxygen (i.e. the O2 flows through every mask regardless of number of pax) or demand ( pax are briefed to pull the mask to initiate flow). Perhaps a QF eng. can state which system VH-OJK has? If it is the latter, then some pax reporting a lack of O2, may not have pulled the mask.
On depressurisation or activating the Pax. Oxy switch in the cockpit, an automatic announcement is played and repeated over the PA system. While it sounds loud enough during Cabin Crew pre-flight checks, it may not be very loud during the actual event - perhaps not sufficiently loud enough to get the attention of confused and frightened pax.