I can only speak to my limited experience with editors hired by digital first publishers. Their job, in the most simplistic terms, is to make your book shine. The ability the author has to reject all the editor's suggestions, none of them, or somewhere in between depends on the publisher, I suppose. I was never forced to accept any editor's suggestions. If I didn't agree with something they changed, or suggested I change, I put a polite comment in the margins, addressing why I wasn't changing it, and went on my merry way. At some publishers, I imagine, that might be a deal breaker. Most contracts speak to the autonomy an author has in terms of both cover art and editing. If they don't, the author needs to clarify that before they sign.
It's tough to say from this one isolated (and bitter) incident whether the book would have sold as well or better had the author been given free rein to use the voice he originally intended. To be honest, I couldn't see much difference, other than narrator, from the one passage. Publishers generally know what will sell at their houses, after all, which is why their editors make the suggestions they do. But editors are only human and they have egos, too. They have particular likes and dislikes. Some of that invariably comes through, although it shouldn't if they're actually doing their job instead of trying to feed their own egos.
I've had really horrible editors, in terms of them missing multiple mistakes that slipped past me, and in terms of them rewriting every sentence to the point my voice was gone, and the manuscript read like a computer program had spit it out. The former I complained about until I had new ones. The latter I rejected every comment and firmly requested never to work with her again. I also found out after the fact she had done the same thing to multiple authors, and was subsequently fired.
My current editor at Evernight is actually a former member of the "old" Litopia. She's not perfect. She misses a lot of little things, but she is very good at characterization and motivation issues within the manuscript, so we're a good fit. I'm sure my experience would be vastly different with a different publisher.