Should be no issues with the Australian routing,
maybe the red flags have something to do with bad dates or misconnection timings

Australia is similar to the US in that you can only do one transcon. ie can't do SYD-PER-MEL, although you can do SYD-PER-ADL/AYQ-MEL

IF you fly SIN-SYD/MEL then the above, then SYD/MEL-LHR you can get two trips on the A380 (although you are adding a segment)


From a mileage-run perspective, you are also not taking advantage of the F upgrades in the US - you would have 6 US segments to maximise, but all depends on your objectives.