I think it's interesting that the liquid rules don't apply to domestic flights in Australia.

Yes, it's inconsistent.

the reason the headsets are collected before landing on Qantas international flights is that the ground cleaners are contractors, so it costs Qantas nothing to get the crew to do it, on domestic flights the cleaners are Qantas staff..mostly..and the turn around times/ flight times are shorter. The lights out before landing is for safety, if there is an emergency evacuation, the crew's eyesight needs to be adjusted to outside to check emergency exits before opening. I worked for 'em for a while