Can Eskimos be regarded a distinct species from Kalahari Bushmen based on morphological differences & geographic isolation?
I think that species delimitation is not a science, or at least not a coherent one. Different fields use different criteria, for practical or historical reasons. For instance, in plants, loads of fertile hybrids exist between species. It seems that botanists do not split based on the traditional biological definition of what a species is: they apparently tend to split more (see the splitters and lumpers notion).

My intuition is that humans are generally interfertile. There even seems to have been interfertility between H. sapiens and H. neandertalensis which have been (or maybe still are?) considered as distinct species. But how are the actual genetic exchanges across the human populations?

In my opinion, if it were proven that genetic exchanges between human populations living very far apart and having evolved very different ecological adaptations (like populations from the arctic and from the Kalahari) were almost reduced to zero, then it would not be absurd to call them different species. Humans would form a continuum that can be considered a unique species (if interfertility is to be taken as a criterion), but with isolated extremes being in the process of forming separate species, somewhat like the extremes of a ring species. Maybe there even is some degree of reproductive incompatibility between very divergent human populations. The thing is that it is not ethically acceptable to experiment with this. We need to rely on "natural" experiments happening. Globalization could potentially tell us in the future, by facilitating encounters between individuals from very distant populations, whether such reproductive incompatibilities exist or not. That said, I doubt that the origins of parents are recorded in a scientifically usable manner when a couple has problems having children.