I have had MRU confirm a booking and get it priced in around 48 hours, allowing for time zone and weekend lags. I'd think a week is quite doable. I would make the initial routing as simple and straightforward as possible, in order to avoid delays caused by BA tariff people contemplating the rules and throwing the bones. If you have to re-issue it later for tweaks, so be it.

Just as a note on good customer relations, I got an email today from BA in MRU advising me that there was no time to lose in booking any new RTWs due to the forthcoming reduction in maximum segments.

If you reconsider and decide on a DONE5 instead of A, be sure to look at prices ex-South Africa, which are generally cheaper than ex-MRU. Plus the AA GSA in Cape Town is very efficient and quick, and you'll save money on fuel fines and nuisance fees by issuing through AA rather than BA, CX or QF.