Turns out it was agent incompetence at work, despite the fact that I had been kind enough to do all the legwork down to identifying the available segments!

I dialled M&M Delhi again, and the second agent set up an entire BOM to CPT business class round trip for two, with a JNB, stopover in about five minutes. It does tick me off that a less aware person might have been intimidated into not going back to M&M and thereby missed out on available seats.

It worked out to just under 2 cents per mile for that routing, and about 1.3 cpm vis a vis a slightly less preferable Ethiopian Airlines itinerary with the JNB to CPT segments in economy.