I don't think you can even issue the ticket unless all segments are confirmed. If that waitlist doesn't clear and your ticket isn't issued, you can't even start your journey at MEL.

If you aren't willing to chance it, book MEL-SIN one way, and HKT-SIN-MEL since it's available, then book a separate SIN-HKT ticket. No stopover restrictions to worry of since you'll be on separate tickets.