The rules of the Oneworld Exporer ticket are here

Rule 2(b) says "Travel must be in a continuous forward direction between TC1 - TC2 - TC3"
Your suggested itinerary violates this rule: your flight LHR-SYD takes you from TC2 to TC3 and then your flight SYD-JNB takes you back to TC2 (and then JNB-HKG takes you back to TC3 again!)

The only way I can think of to get both JNB and SYD-LHR (or v.v.) into a valid itinerary is to make use of the rule (Rule 2(e)3) that allows a transit of Europe to get to/from certain African countries. This will work:
KRT-AMM-LHR-SYD-HKG-LAX-JFK-GRU-SCL-xLAX-xLHR-NBO,JNB-LHR-DXB. But you'd have to make your own way from NBO to JNB

You are allowed only two stopovers in the continent of origin (Rule 6). But you are allowed whatever number of flight segments is allowed for the continent (in the case of Europe that is 4 - Rule 2(h))

My suggested itinerary has 14 segments, leaving two more to use (in true FT spirit, one mustn't waste these!) Also I'd suggest HKG-JFK-LAX instead of HKG-LAX-JFK - more time in CX F and of course miles earned. Maybe ...HKG-xJFK-GRU-SCL-LAX-JFK...

Mileage Monkey is good for playing around with itineraries