Just back from 3 weeks in Zimbabwe.

Vic Falls is perfectly safe and gives access to the 80% of the falls that lie in Zim. I'd strongly recommend considering flying in there and then crossing the border to Zambia after a day or two.

We flew LHR-JNB-LHR using a 2 for 1 (out in first, back in club). We couldn't attach Comair flights to the booking. Even in economy I decided that 10K miles plus taxes was good value compared to cash for the homeward HRE-JNB leg. Availability was good, in economy at least. We just had 3 hours at JNB and it was no problem to check our bags right through.

On the outbound we flew SAA from JNB-VFA as it was cheaper, and we discovered another bonus of taking SAA - the visa queue was incredibly slow, and they can still be processing SAA passengers when the Comair flight arrives.

Be aware that the middle seats in economy on the Comair planes are very narrow indeed. Next time we'd reserve two aisle seats.