My approach would be to arrive in good time for the South African departure as Comair will almost always rebook if there is space. So I, personally, take the risk every now and again even with an unchangeable ticket.
I personally have only seen the Comair cancelled on the odd occasion in the off-peak tourist season (peak is June - October in Zambia). I have seen it run late though (up to about 2 hours) in the recent past. Comair sometimes take the flight to a JNB-VFA-LVI-JNB circular and I've never seen that one cancelled.
The general gist would be that it's not foolproof as with any connection on two tickets but overall I think it's pretty safe.