For South Africa you can use the SWU's to upgrade your transatlantic flights to MUC, LHR, or FRA (not sure about ZRH since it is now operated by UA metal). I am not sure if you could use a CO issued SWU on LH for the FRA - JNB flight (maybe a MUC flight also that time of year), but even if you can it would be another SWU and not the same one as you used for the transatlantic flight. So for 2 people it would be 4 SWU's each direction.

If you want to use the 1 SWU for the entire direction Japan would be the better choice. Not sure the difficulty in using SWU's that time of year.

Having said that SA is a great place with a lot of cool things to do. It is hot that time of year though.