Is your CPT-JNB flight on SAA? If so I think they should be able to interline your bags for you as my mom often does PLZ-JNB-ATL on different tickets but they still interline. I am not sure about BA or the other domestic airlines however. If your on SAA I'd get them to interline them in CPT, but you may need to ask to speak with a few agents before you find someone who is able/willing to do it.

2hr10min may be tight. Luggage claim should be quite quick but you never know. Immigration right before that evening bank of flights may be a bit busy but security and immigration I can't see taking more than 30min. I think the check in counter to drop off your bags though will be the wild card. I haven't flown DL out of JNB before but if its like SAA or LH can it can sometimes take a while even if just a few people are in front of you in line.

Personally I'd change to have a longer layover so I am not worrying about the connection on the way from CPT. Hopefully the domestic airline will be sympathetic if its a restricted ticket...

Good luck