The primary factor that keeps most tropical areas of Southern Africa cool is 2) High elevation.
While the Benguela Current does bring cooler ocean temperatures along the coast, the high elevation of regions such as the Drakensberg Mountains and other plateaus contributes significantly to lower temperatures in those areas. Monsoons are more relevant to other regions and not typically a strong influence in Southern Africa. The harmattan refers to dry and dusty trade winds that occur in West Africa, which do not impact Southern Africa significantly.