Debate
Or maybe we’ve just somehow moved from a relativistic few decades, where it was more common to assume that there were good things to be said for all sides of a given argument, to a polarized, judgmental culture where trying to see the validity in perspectives you don’t share is treated as an intellectual and moral weakness. 
Plenty of battles do have to be fought, not bothsidesed. There are good reasons our culture has shifted away from cozy relativism and picked up versions of the old puritan/ crusading/ inquisitorial tools again. But applying that mindset to interpretations of choose-your-own-adventure books? I mean, in fiction, usually there really is something to be said for both sides. (For arguments that real people actually have. I’ve never heard anyone argue that LotR is pro-factory, and I’ll bet you haven’t either.)
When all the strawmen are lying dead round the back, we can get back to the ideas actually being shared on the thread. I mostly share your opinions and ways of reading… but mistaking opinion for fact is an intellectual sin at least as serious as treating all opinions as equally true.
One foundational idea you keep coming back to is that the characters in one playthrough are simply different from the characters you’d meet in another, that horny NPCs B and C “are not the same B and C from the playthrough where you’re romancing B.” That’s a totally plausible way of reading IF, where each new reading creates its own new self-contained universe with new characters… but it’s not the only “objective” or factual way.
And I’m not sure anyone actually does it consistently, whether as a reader or a writer. It happens to be my way of reading romances in games, too – but it’s not how I naturally read all aspects of IF. If I uncover secret evidence that B is a psychotic murderer, I’m probably going to take that as an important fact for the character across all playthroughs, and be disoriented (if not annoyed) if on another readthrough B turns out to be a kindly soul who’s supposed to be my best friend. If I find that whether I choose to walk east or west, I end up outside the same house, I’m going to feel railroaded; I won’t shrug it off because each readthrough creates a new and independent universe.
I don’t think there’s anything wrong with the inconsistent experience I just described. I could probably come up with a detailed justification for it if I wanted to – a reason why in some cases it feels right to me to treat different readthroughs as explorations of a single universe/character, and in other cases it feels right to treat them as new, alternate universes/characters that don’t inform each other. But that probably wouldn’t be adequate to explain the other inconsistencies in my experience of IF… for example why I sometimes enjoy and am sometimes annoyed by being given the choice to determine something in the gameworld that the character would in reality have no power over.
At the end of the day, I bet that if we dig down in just about anyone’s experience of fiction, we would find similar types of unresolved inconsistency. I think that’s just the nature of something that we inevitably experience as real and not-real at the same time. And I wouldn’t expect to conclude that on this, one person’s inconsistencies are “truer” than someone else’s. With fiction, there just isn’t
Similarly with “who’s the first choice, who’s the second choice” – my instinctive response would be like yours, sure, the MC is the NPC’s first choice. But I also recognize that when we drill down into it, talking about NPC choice at all is problematic. We all know that it’s really the weirdly omnipotent MC who’s being given all the agency here. We have meta-knowledge about ROs and the way a game is going to “offer” them to us, as Chani said.
Personally, my own immersion at this point generally isn’t affected by that meta-knowledge. I don’t feel that the choices an NPC is portrayed as making while not under the influence of the MC-RO vortex of destiny are more authentic, revealing their “real” preferences. But when I read someone else who does react that way under the influence of meta-knowledge, I don’t think they’re objectively wrong to do so. All fiction is experienced in a meta-fictional frame, and how much of that frame we ignore and how much we carry in with us just isn’t an objective, “square-with-three-sides” situation.
Finally on “canon” and authorial intent… saying “the author clearly meant/preferred this” is risky and presumptuous, which is why I’ve been encouraging Chani not to say it. But saying “it feels like the author meant/preferred this” or equivalent is an almost inevitable part of any critical reading… and so many people mean that when they say “canon” that I’d rather engage with what they mean than quibble with their word choice.
Don’t get me wrong: like Malin, I find the whole idea of canon odd and questionable in IF, at least the majority of games where the author has written a bunch of equally valid options. I know some authors like to develop “one true playthrough,” but most don’t. For them I wouldn’t say any single plot train is canon.
At the same time, thinking about criticisms of my own game: I really did put a huge amount more care and craft into the Breden-MC romance than the Breden-NPC romance, and you can get further in Game 1 with your relationship with Breden than with any other RO. Plenty of readers have accused XoR of having one canonical RO as a result.
If by “canon” they meant “explicit authorial intent,” they’d be mistaken. It certainly wasn’t my intent that Breden should be considered the game’s One True Romance, rather than an interesting nexus of early-series plotlines and experiences. But I don’t think they’re using canon that way, because I don’t think they’d drop their criticism if I explained my actual intentions: “You may not have meant to write a canonical romance, but you did anyway.”
I’d call what they’re describing implicit canon. Like “official” canon (and unlike headcanon) it comes from the author, not the reader. Unlike official canon, it may not be something the author explicitly intended, and indeed might be something they’d want to disavow… but the author wrote it anyway, in this case by putting more care and effort and rewards into one plotline/character than another. I think that’s a legitimate critical interpretation of a work, and arguing that the critic shouldn’t use “canon” because only I as author get to decide what’s canon feels both kinda useless and like an evasion of the actual point.
Bringing it back to the NPC-on-NPC action: it seems clear that for some readers who have a strong sense of the MC as their character, the plots an author writes with the other characters can seem like they have more (implicitly) canonical status. Those are the characters that belong to the author, and what the author does with them is more canonical than what the author writes about the MC, who is mostly owned by the reader. That’s not my reading or how I’d be inclined to use the word… but wrong? Nah.
At the end of the day, I’m with Malin that readers can have valid feelings that authors decline to take into consideration – there are too many potential reader-feelings, many of them in direct conflict with each other. But I’m not going to add insult to whatever-level-of-injury-someone-takes-from-a-mobile-game by telling those readers that their desires are invalid or their way of experiencing a game somehow objectively wrong.
Pfffft, as if a one-line aside in a PS to someone else was me arguing with you? They don’t call me Haverstone for nothing. 
You noted that you’re not speaking for all authors here, and I appreciate that. My goal, even as someone writing a story to sell, is to end up with something that I enjoy. If I try instead to take an imagined audience as my main point of reference, I’ll lose focus and energy. There are too many potential audiences out there, all craving different things.
I’m happy to write in community; it’s made my writing better, i.e. more satisfying to me. But I decide what feedback to accept in light of my goal to write something I’m really satisfied with. If I succeed in that goal, I trust it’ll find an adequate audience.