We have sailed Darwin to RSA, South of Madagascar. 26 footer. No problems.
We sailed late (September-November), for two reason's: we had to time it with our earlier sail from New Caledonia
earlier the same season, and 2) the peak winds are mid-ocean from July to August - and sailing a smallish boat we tried not to be there then.
Indeed, even so late in the season we lost
the trades S of Madagascar but nonetheless sailed on without any problem to South Africa. The landfall (ours was Richard's Bay) is where you have to read the weather
well - to avoid being caught in the middle of the infamous Aghullas current
while a SW system approaches - not the combination you will like - no matter what size the boat.
The other route - via the Mozambique Channel - we cannot recommend - it can be upwind on your landfall. But it can be done OK if you want to visit places thereabout.
We did not use Cornell's book which we found inadequate earlier, we used the Ocean Passages of the World and the Herrick's book. We found both very useful.
We used RSA weatherfax to time our crossing of the current
Some cruisers stopped at Chagos
but now I hear one needs a permit
to be obtained in advance.
Some other cruisers stopped at the Cargados Carajos and other shallows in the area and loved it there.
Otherwise the Chrismas, the Keeling (only allowed to stop in the Southern), Mauritius, Reunion, and a couple of minor specks - all very lovely, great nature, nice people, zero red tape