Of course this route isn't an RTW - you don't cross either the Pacific or Atlantic.

You might look at a "Circle Indian Ocean" fare - as of last year, a business class 3-continent "DONEWCX" was a bit cheaper than a 4-continent DONE4 RTW is now (ex-Korea) but that may have changed. Aside from basic return fares (or openjaw or point-to-point ones) the Oneworld Explorer might be cost-effective, but you can't get everywhere you want - for the moment MRU is a "dead-end" - no intra-Africa routes (yet) approved; and no OW service to the Seychelles from anywhere AFAIK.

You could make the RTW work by going ICN-HKG- North America - LHR-JNB-DUR-JNB, then JNB-MRU either on your own or as part of the RTW - BA is supposedly going to serve MRU from JNB starting sometime this year. From MRU, buy a separate ticket to SEZ and back, then MRU-JNB-HKG-BOM-HKG-ICN.

So, say you pick JFK as your NA city, it would be ICN-HKG-JFK-LHR-JNB-DUR-JNB-[MRU-(SEZ)-JNB]-HKG-BOM-HKG-ICN.

Maybe others have better ideas.