It is unclear how the OP could use the return leg of the ticket without using the outward leg. Strange that CX let him do that. In a sense, they have been quite accommodating. But it is hard to claim the reimbursement of part of that ticket once it has been half-used.

I fail to see why CX should reimburse the QF segment that the OP bought on his own will, without even considering alternative reroutings that CX would offer. Unfortunately long delays, even cancellations, are common from/to PVG and it is not uncommon that CX has to reroute pax with a connection in HKG. There are several daily CX/KA flights PVG-HKG and HKG-SYD. It is probable that the OP bought a cheap ticket on CX and that is the risk of misconnect.

On the topic of travel insurance, I think that the coverage of such an incident depends on the insurance contract. The question is whether the insurance on the purchased CX ticket would allow for the payment of the cost of the one-way QF ticket without the OP having even tried to be in touch with CX regarding alternative arrangements. But that is not the issue at hand anyway.