Hi
I've just come back from this trip, but without the Zambia add-on. What happened with our group was that the Zambia people were taken to their hotel on the way to drop us homeward-bound people at the airport. I imagine they would have been taken direct from the hotel, which was near the airport, straight to the airport for their flight the next day. So they would not have had to go into the centre ofJohannnesburg at all, or even been near it.
Best wishes for your trip. You'll pack a lot in and will have some early starts - but worth it!