With speculation...
I would suspect that P1s would be the least likely to be selected in general
I would suspect that the airline would give longest time possible for a P1 to make a connection without being too late to be able to process standby passengers
I would say that at 15 minutes before departure. I woudl find it very surprising if they hadn't processed the offloading
Also - it may not have been LAX that did it ; it could have been the P1 team actively monitoring the P1 travel that saw the state of connection and took it upon itself to minimise the disruptiuon by booking the BNE flight
What I would never expect is for QF to have passengers standing by for seats and for QF to leave with empty seats on the offchance that the P1 members had got there in the next 5 minutes
Looking at a boarding pass that I have for tomorrow ( not LAX admittedly ) , it states gate closes XX:XX where this is 20 minutes before departure. I doubt very much that the stated time is less than 20 minutes at LAX
I think that anyone blindly paying extra to travel on Qantas to get to P1 status is misguided - the reality is that loyalty is purely one way; QF wants the loyalty of a dog from its members whilst it knows that they are simply one of many in the database to stroke from time to time and tell them how special they are