Significant characteristics of major characters should never change. It's disrespectful and not racist to oppose it.

REPEAT!

Nope. The most one that auditions the best should get it. Stop demanding for affirmative action.

Wait, isn't there a Napoleon film or movie upcoming? If I have to guess, I would say Napoleon's role will be played by Kevin Hart, because Hollywood is really worried to represent Napoleon's real height.

I agree that significant characteristics are usually important to retain, but then you went and listed a bunch of typically insignificant characteristics. Spider-man's skin color is immaterial. His defining traits as Peter Parker were his unpopularity, brilliance, and money woes. Everything else is window dressing, and the story could be told just as effectively were he of another race. Same goes for your mermaid. She may have been described that way in the book, but her physical description is in no way fundamental to the story. She was described that way because it's a Danish book, and she was written to look Danish. Absolutely nothing of even the slightest importance changes if she becomes African, Asian, or any other ethnicity.

As for actors-- who cares if Jake Gyllenhaal isn't Persian or Christian Bale isn't Egyptian. It's acting. Marlon Brando wasn't Kryptonian and he was fine as Jor-El. An actor is ACTING, and we suspend our disbelief and accept them as the character. All that matters is how well the play the role.

"her skin was as clear and delicate as a rose-leaf,"

so like transparent skin?
Which actor will that be?
looks like they'll have to cast a real mermaid.