It is a fantasy world that was flat until the Second Age in order to prevent men from accessing Valinor. He regretted not changing it to be round from the start but never officially changed it. Polar, Equatorial, etc don't apply because it wasn't used as a basis for things in Tolkien's lore. Even then the story doesn't change because different skin tones are present. "Nature's Way" doesn't exist. Evolution wasn't even really a concept because the races were directly created by the Gods.
A black Frodo changes nothing does it? He would still be the same character but with different color skin. None of your explanations actually cover how the story is different because of non-white skin tones. Strange, right? Backgrounds, societies, the entire world, etc do not change just because black skin exists.
It can be as simple as Eru Iluvatar made it possible because you know he is the one that created everything with the help of the Ainur.