The Oneworld tool is very buggy and not to be relied upon. I'd suggest investing in a subscription to Expert Flyer, where you can see availability by booking class on any flight. JL 52 looks like it has lots of D seats available in late February.

For JNB-MEL, I'd probably go with CX via HKG to reduce the YQ fees, but if BA is issuing the ticket in the first place, you'll be fighting an uphill battle as BA whacks YQ on most everything.