1. No. The requests are independent of each other. The date of request is only used as a tie breaker within the class you've booked so the majority of the time it's not even looked at. Someone booked in a higher class than you, of equal status, will have higher priority to clear. Eg. someone...
The ticketing robot takes about 5-10 mins to issue a ticket for any carrier. So if it's any longer than that, either the robot rejected it, or there's something else wrong with the PNR.
This loophole exists in the old reservations system that up until recently was the main system in use, however they've mostly transitioned to the new reservations system now and the new reservations system does not exhibit the same behaviour.
https://www.australianfrequentflyer.com.au/british-airways-limits-partner-reward-seats/
Nothing available on PMO-LHR to even BA members, but CTA-LGW is available to BA members but not QF/partners.
At other QR tag flight ports, they check everyone's boarding pass as you disembark and if you weren't terminating at that port you were directed to a separate area. I don't believe that they have implemented this at MEL, but they could very well do so.
For QF28 to QF5 you'll clear immigration / collect your bag in PER. SYD international transit is open all day so it is permitted.
Transiting internationally to QF9 however varies, as this is due to the limited international transit hours at MEL.
If done correctly, it does allow some limited benefits to flow to the other PNRs, eg. accessing preferred seating rows. It's also important for PCV reasons - the linked booking takes on the PCV of the highest PCV pax.
PCV is influenced by a variety of factors (in no particular order) such as:
1. Meet and assist or wheelchair assistance
2. RBD booked
3. Status
4. Pax in your travel party
5. Commercial value to QF