Wow! Concorde is an AUD 2850 surcharge on RTW fares, so being Plat is over AUD 5K benefit to you. And the infant fare is another 1K saving ... you have done well in that respect. That 35K mileage limit on QF is very mean (AA allows up to 50K miles on Oneworld awards, but for greater cost). These other rules or exceptions make up for it, quite a tangible benefit. I'd suggesting flying BA LHR-HKG and then CX HKG-SYD. This gets you the BA lounge service at LHR T4 (Moulton Brown spa and Concorde room) which you wouldn't be able to access if departing on CX; and you can compare to the Wing/Pier lounges in HKG (maybe the best F lounges in the world, certainly with the best food). Plus the shopping in HKG is better (duty free can be 30-50% lower than other duty free cities, at least for the things that I've noticed). VS flies to HKG and BA is on its best behaviour on the HKG route (using better equipment, for example). And this routing leaves the best service for last (I think you'll find that CX is a notch or two or three better than the other airlines on your trip).