Two quick points...
i) I don't know what sort of prices you've found, but JNB-PLZ,CPT-JNB isn't necessarily two one-ways. The SAA website does have a multiple cities option and will allow the open jaw. Both options (JNB-PLZ-JNB and the open-jaw) seem to come out around ZAR1000 pp. You also might want to try and group the domestic legs with your int'l flights, depending on whether they are an associate of SAA.
ii) You might want to try Orbitz for the one-way car hire if you haven't already. It will compare the operators and the key line is total price which includes one-way fees. One-way fees sometimes depend on the car class so this sometimes brings out a good deal (though once you know the deal, I tend to go to the Avis/Hertz/etc site and try and book it there).
Hope that might help.