Hrm. I wasn't really wanting to get into more detail on this, but I feel I must now. My own fault for starting the thread I guess!
Make them telepathic, give them a personality disorder (so they can't even tell themselves apart) and tell everything from their point of view, which is a joint point-of-view and therefore only one. There. I'm good at solving problems, don't you think?
I realise you're being jokey, but this is already sort of the case. One twin (the girl) suffers from hallucinations and delusions and is an unreliable narrator; the other (the guy) doesn't, but is unreliable in other ways (his self perception as a social "paragon"). The emotional crux of her arc is discovering this and working through it.
The girl character effectively dies at the end, though not straight away, and is revealed not to be the chosen one that she was set up to become. This is also the character the editor is suggesting making the sole pov.
Seriously, though, you can characterise people through what they do and how they're perceived by others, while keeping their inner workings a secret (unless they reveal it themselves). Important characters in a novel don't necessarily need a point of view, not even protagonists. Characters will look differently to readers, depending on the point of view character who observes them. Someone who knows them well? Surprises will have more emotional impact. Someone who doesn't know them at all? The mystery is heightened. A naive point of view who doesn't get what every normal reader can be supposed to understand? You have dramatic irony. Wanting us to understand a character isn't necessarily a reason for their point of view being mandatory.
The reason I want multiple povs is for precisely many of the reasons you list. The story is about perception, or supposed to be; the ways in which we miscommunicate, misunderstand, and misinterpret reality.
As above, the twins have their own points of view because they equally carry the burden of being join protagonists. Their personal arcs are emotional, because they do not actually know the plot which is occurring around them, an d have no way of finding out.
The plot is therefore carried and dealt with by the two adult Calaani who have povs. Here is another issue of misunderstanding; I think the editor didn't realise they are and would continue to be, major characters in subsequent MS. Either way, removing those characters would remove the plot.
The final pov only has three chapters which I don't think is extravagant, mostly occuring early on. She's half human (none of the other POVS are human) and more relateable; she is therefore able to explain to the reader things about the world and setting which I'm not sure could be introduced otherwise. This pov could be eliminated, but I'm not sure much would be gained; it would only be more awkward to introduce her later on, in sequels.
I say that, though, as someone who loves multiple points of view when reading, and whose whole reason to write (when I still was writing) was to contrast as many points of view as possible and create a social story from that mess. 5 points of view? I laugh at that puny number.
Why am I posting? I don't think cutting points of view is the same as cutting content, so this line surprised me. If this line surprises me, then maybe it would surprise your editor, too? (Although I'd expect an editor to have worked with more people than I have [as a beta], and thus be more experienced with potential miscommunication.) Basically, there's a possibility that you're seeing bigger changes than she might have intended to suggest.
Well, this comes back to her assertion that the story's central question is MC's quest for ascension (coming of age). I don't think that's accurate at all; the central question is who are her people/race. She would like me to take out exploration of their society, but exploration of their society is sort of the point.
Also, it strikes me that Old Hack talks in terms of problems and fixes, while you talk in terms of changes. Are the changes supposed to fix a problem she pointed out at all, or is this just "to make the book better"? What's the problem with your book being adult fantasy rather than young adult? What's the problem with 5 points of view? The question really isn't what the text you submitted could be; it's what's wrong with the text in the first place. "5 points of view --> 1 point of view" might be fixing the lack of focus for your text, or it might increase the identification with a single character, or, or, or. If you see the problem your editor is seeing you can find your own fixes. If all you have is a suggested change you can take it or leave it, and that's about it.
I think that's my main complaint. The questions you mention aren't addressed at all. If they'd said, "this cannot be adult fantasy becuase you have X, therefore it is currently stuck as YA" then I'd have something to work with. At the moment they're just "here are things you can do better in your MS", but all the things they suggest doing are in the context/assumption that I'll be rewriting as YA. I think they genuinely don't see any value in aiming for something other than something other than YA.
again, I accept that the MS isn't working for her, and that's *well* worth looking knowing, but my inclination is that it'd benefit from having the YA aspects scaled back, not blown up into a different MS. At the very least we're not on the same page, much less the same book.