Nemo was indeed originally written as a former Polish rebel against Russian rule, but Verne's publisher objected on the grounds that this might offend Russians[1], and so the published version of Twenty Thousand Leagues had no mention of national origin or ethnicity for Nemo.

[1] Both because the Russian market was important for sales of French books -- and government censorship in Russia was a strong possibility -- and because the publisher supported France's ongoing negotiations with Russia for a potential alliance, and worried a book promoting anti-Russian themes might make the negotiations more difficult.

The change about Nemo's origin must have been done before finishing "Twenty Thousand Leagues Under the Seas", because already in that there were expressions of his discontent with the British Empire, above any others, I think towards the end of the novel, when he attacked some British war ship (an attack that was used to convince the more reluctant of the prisoners that they must escape from the submarine).

That novel has been published in episodes scattered over more than a year, so it would not be unusual if towards its end Verne had a different vision of the story line than when he had started writing it, one year before.

Sorry, back at my computer now.

When it comes to ethnicity / race (or really any characteristic) there's a few ways writers can do it:

1. Colourblind - race has no real impact. It's simply not a big deal when writing the character or story. Nemo was originally going to be Polish, but he ended up Indian. The only important thing about him being Indian is that he opposes the British Empire, but really anyone can stand up against injustice. An awful lot of characters are actually like this, because they are mostly written to serve the plot, so as long as Nemo is smart, brave, cultured, loves the sea, tenacious, and stands up against injustice he can be anyone.

2. Tokenism - it effects the character, but not the story e.g. your plucky group of teenage crime fighters has a black kid who is street smart, and an Asian kid who is book smart. Or you could be cool and reverse the stereotypes. I'd say it's fine, as long as it's not too ham-fisted for whatever the current year is.

3. Make the entire plot about this characteristic, because you want to tell an "authentic" story.

So does race effect nothing (substantial), the character, or the story itself? For Nemo, I'd say his race is absolutely not a big deal at all.

See e.g. the debate on colourblind casting. https://en.m.wikipedia.org/wiki/Color-blind_casting