Your Asia/Australia construction looks like it follows all the rules. I don't know how Cathay would deal with it, but my experience has been that all their rate agents know the rules and interpret them fairly so you should have no problems.

You're right about the Africa-Europe dilemma. If you want to include Africa, best do it at the point you were planning HKG-LHR. Substitute HKG-JNB-LHR. If that doesn't work for personal reasons, know that there are a couple of African countries from which you can transit Europe. I never remember which ones since I've never traveled to either, and there is no OW way to get to either from any other African OW port. (Nor any particularly attractive fares on other carriers.)