Some comic-book events feature a black Superman, and that raises questions about whether Kryptonians coincidentally match every ethnic group on Earth

Not exactly, but they have dark and light skin. Krypton's red sun is a big deal in Kryptonian health. Kryptonian communities living closer to the equator would probably evolve higher melanin the same as humans did. Actually, light-skinned Kryptonians require a bigger suspension of disbelief than dark-skinned ones do.

And obviously the racial dynamics between dark and light Kryptonians wouldn't be the same as on earth. It would depend upon factors like the shape of continents, the location of arable soil, of large herbivores and of predators, and of minerals.

Fun fact: swords made out of meteorites are a common trope in Japan, because Japan has poor iron and big mountains.

Some comic-book events feature a black Superman, and that raises questions about whether Kryptonians coincidentally match every ethnic group on Earth, or if they’ve got some skin colors that aren’t an option for humans, or if there’s some imprinting transformation wiggle-room when they show up as babies on rocketships.

With Kryptonians, I think if you can suspend disbelief enough accept that a planet in another star system could evolve people that are identical in appearance to humans, then you kinda have to accept that there would also be similar ethnicities there too. I mean it seems impossible someone from another planet would look exactly like a human, so you have to be willing to accept the impossible for Superman to work at all. Also Kryptonians are somewhat an metaphor for Jews and there's black Jews, so there should be black Kryptonians to fit the metaphor.

And for the constant murmur of Idris Elba maybe playing James Bond, I have to point out the sliding window of ambiguous canon.

Yeah I feel like James Bond is operates under comic book time. So yeah it's the same character, but part of the suspension of disbelief relies on accepting that we just aren't supposed to think about how this guy doesn't age. So the same character changing ethnicity feels like it's it's either stretching that suspension of disbelief too much or straight up abandoning the comic book time continuity aspect of James Bond. Either way there's going to be people bothered by it and it's not necessarily a racist thing, it's just not liking that an aspect of a franchise they enjoy has been changed.

Don’t ask what movie Bruce Wayne and his parents were seeing

If you want to feel old, Bruce Wayne could have watched one of the Antonio Banderas Zorro movies as a child and be in his 30s today.

But… Doctor fucking Who? They’re shapeshifters. Every single Gallifreyan gets a dozen chances (or a dozen ish, evidently) to completely scrobble their appearance while maintaining a continuous personality. Even their fucking accents can change.

Hey... lots of planets have a North!