Thanks for the speedy replies.
I'm booking this as an award (AA and UA miles for CX/QF and SQ, respectively), so options of which SQ flight to take are limited (can't get the A388 for SIN-MEL).
If I did the CX/QF option, I'd make it into a fun night in HKG for the layover, and it'd be QF F vs. J, at least, based on availability.
Really good stuff here -- thank you!