While it’s perfectly possible for someone of Navajo descent to find themselves in central Africa, it’s not really that likely.

What part about literally any story about heroes and adventures is "really that likely"? Every story ever told is told because they're unique and thrilling and unusual. Pretending like your problem with the "wrong" races mixing in fiction is because it's "unlikely" belies the fact that everything in these stories is unlikely. Why aren't you complaining about main characters that are shockingly born from the lost line of monarchs, the last heir able to save the kingdom? Or having a mysterious, ancient weapon literally fall into their hands? Or any other number of preposterously unlikely things that are what make these stories worth telling? You don't complain about them because they don't bother you. But a black person in Scotland? THAT'S where you draw the line? Come the fuck on.