They could've just gone with the original ending where Kuze and Mira fuse, but they didn't because it didn't test well. So they went with this ending that literally tells us that this character is actually Asian.
They literally kill two Asian people and put them in the bodies of white people.
I don't know why the first ending didn't test well - I don't know if they've even confirmed what that first ending was (even though you're speaking of it in definitive terms). It could be that the fusion wouldn't make sense because she originally fused with the Puppetmaster, an AI, which could theoretically exist within her cybernetics. It makes less sci-fi sense, and there is no precedent, for a human brain to fuse with another human brain.
You keep bringing up the same talking points about ScarJo's statement, and while it sounds like a great 'gotcha', the context is clearly that she is not playing another race - such as the guy from Cloud Atlas playing an Asian, or Emma Stone being passed off as authentically Asian. ScarJo's "character" in this movie is a machine with a biological brain that has been wiped. But because this is 'new tech' in this bizarro future, parts of the brain's memories are leaking through, like data that hasn't properly been zeroed out in a wiped hard drive. Her character is still Mira Killian, a machine with a human brain. The identity known as Motoko was killed when they wiped the brain. She had no emotional response to Motoko's mother, she only let her know that her daughter no longer exists but has given rise to a new lifeform that still holds similar ideals. (even though the audience knows they were artificially placed there) (Edit: The only emotion she shows is towards Kuze, but as the movie shows, she doesn't care because his brain used to belong to Hideo, but because she finally found another life form that is going through the same struggle she is.)
This is the cornerstone of the Ship of Theseus thought experiment - when all parts have been replaced, are you the same person. All of her parts were replaced. When they ask her at the end of the movie if she consents, she doesn't say she's Motoko, she says she's Major. She is acknowledging that she is a different being.
Was this twist necessary? I don't think it was. I think it was put in as a weak wink-wink-nod to fans who can't reconcile the anime and the movie being different. Does it seem like whitewashing when there's not two hours of context put to it? Yeah, absolutely. And they should have been cognizant of this inevitable backlash, but they were going to get backlash either way. They were basically fucked no matter what they did.