All sorted in the end: went with the Easter departure. Ticket is rated and Mindpearl will issue tomorrow. Taxes came out a bit high ZAR8640 (19%), but considering that I have 2 LHR stopovers (technically one is just a transit, but open-dated), I guess to be expected.
One question. If one of the LHR stopovers becomes a transit, do I get a refund on the APD? This will definitely happen with my BKK-LHR-DAR legs as I need to transit within 24 hours.