most of your questions i cant answer,
however welcome to flyertalk and the BA forum FlyingVet
As for your 2for1 voucher and comair flights, you can only use your 2for1 voucher for BA metal operated flights, so your limited to flights ex LHR, LGW and LCY.
you might be best booking your BA flights using your 2for1 voucher and miles and then just booking your one way with BA paying cash and earning a small amount of miles back, YES you will be able to use the lounge in JNB as a result of arriving in F assuming your onward flight is the same day.
Enjoy the planning etc etc etc
cs