I can't see QF getting too picky about an extra bag or two on SYD-MEL. Just remember the maximum weight of a single piece cannot exceed 32kg due to occupational health and safety requirements. The 7kg weight for carry on is either enforced strictly or not at all, so this can be a bit touch and go. Some flights I get away with over 9kg and others the carry on bag has come it at 7.2kg and has had to be checked.
If you booked your seats on the same booking QF will make every effort to automatically seat you together. Having travelled with the US style of pre-allocation and the Australian style of computerised allocation the QF system works surprisingly well at not splitting people travelling together apart.
I have transferred from QF to AS in LAX (T4 to T3 and TBI to T3) on a number of occasions and this process can take anywhere from 30 to 90 minutes depending on a number of factors. I would probably take the 3 hour option to play it safe.