Thanks so much for all the help!

It sounds like the BA JNB-LVI flight is the better option, despite the ground connection on the other end.

Since my MIA-LHR-JNB flights are already booked as reward tickets (they had no availability to LVI), if I purchase the JNB-LVI tickets separately, will there be any problem checking bags through (if I can't convince wife and kids to use carry-on only)?

Also, our reward tickets are on two different PNRs (because we were using two reward companion certificates); will it make things easier if the JNB-LVI flights are also on two different PNRs?

Lastly, when people refer to baggage theft at JNB, are tey generally referring to entire bags disappearing, or just certain contents disappearing from the bags?