I am a bit confused. Very few white people lived in West Africa before modern medical tech, the European outposts there were always tiny. It isn't random chance that South Africa, out of the malarial zone, was one of the few white settler colonies.
As for slave use....just compare and contrast a climate map with slave usage. Areas with lots of yellow fever mean lots of slaves. In the USA south, for example, they tried white indentured servants but they died at too fast a rate. It was only after exhausting that (and local Indians) they turned to blacks.
South Africa probably was simply more geographically accessible and advantegeous to settle, plus it had a smaller population than either West Africa or the Congo Basin.
Yellow fever was brought with Africans to begin with, plus it would be ridiculous to think that the areas were slave labor is needed to grow specific crops and malaria/yellow fever exposed region are the same.
Indentured slaves are not good when you are trying to create a permanent slave population because that's outside the nature of the indentured servitude, it doesn't seem economically as advantegous to continuously import people, not only because death rates, but also because the end of contracts and on top of that face stiff resistance from various groups in the home country.
Also, again, my point is not that there is no difference in survival rates, but the thing is that we cannot ignore the fact that non-African populations did survive in relevant and even large numbers even before the industrial era, so the reasons for the usage of African slaves are many and different.
Perihelion was always in January, and Southern Hemisphere has less land, thus less dust in the air, so UV levels were higher already before industrial era.
But how much? I see conflicting data on the matter, a lot of maps show New Zealand and southern Australia to be not much different from the Mediterranean region(on average at least), maybe the problem is more the yearly extremes.