You must fly where the airlines fly. If you want to use Oneworld, you can't go directly from Africa to South America. If you start in Europe and want to go to Africa and then the Americas, you have to either go the long way around (via Asia or Australia) or use one exception to the rules that allows you to return to Europe (specifically, London) from either Kenya, Uganda, Tanzania, Ghana or Nigeria (so in your case Nairobi) provided you connect within 24 hours to another continent. So if you can reverse your order of visiting Africa, Joburg first, then Kenya, you can go Nairobi - London (connection) - Buenos Aires, no problem. From there, four permitted segments in South America, then North America (of which Mexico is a part) then there's a Japan Airlines flight from Mexico City to Vancouver to Tokyo, then up to four segments in Asia, done. Note Kathmandu won't be an eligible Oneworld city until next year, assuming Dragonair joins the alliance as anticipated. You'll need to go back to Hong Kong from Kathmandu to catch a flight to Paris.

There are several types of fares in "economy." Seats eligible for RTW tickets are limited in number on most flights, so it's not always possible to get a flight on the day you'd like.

The rules for these tickets are complicated. You'd be wise to invest some time reading the "sticky" threads above, and also the comparable threads on the Star Alliance board.