Q2 - Yes. As long as the outbound is connecting only, your return from Europe may stopover in HKG or anywhere else in Asia. But remember, you've already used up ONE segment for the SIN-HKG sector, so you're allowed 3 more sectors enroute to Australia on your return.

However, QF/CX might be a little bit more picky when it comes time to issue the ticket and they might insist that you fly direct SYD-JFK on QF or at least via the USA instead of routing via Asia.