AL, for the umpteenth time, you've wandered off into la la land.
A creative medium should still be held to the standards of reality.
By "reality", you obviously mean "good writing", which is subjective and might as well be you saying "if it's not what I think is realistic, it's just "creator fan fiction"", which leads me to:
Whether fictional or not it's both selfish and unrealistically entitled of an author to think they can have all the power when they're only Human like us.
The irony here. They do have the creative power to do with it what they want because it's their work. It's entitled to hold the validity of it to your standards. As consumers we're entitled to an opinion, our interpretations, and to either take it or leave it. Anything more, like staking a claim on a piece of art's status, is selfish in my opinion.
No person is ever completely infallible and that applies to fiction as well because no one can handle being infallible.
So, if you're saying fictional canon is defined by accuracy and can be rejected if a creator makes a mistake, why can't we declare 3 as non-canon? Miku seeing ghosts again after saying she didn't after Himuro is clearly incorrect, but I suppose you'd make allowances for that because you approve of the rest of her story?
Considering the subplot was written to appease what they thought fans wanted
I'm not sure where they said that, but if I remember right, Nintendo requested for Miku to return, the Ghost Marriage plot was being written independent to her, and while writing her into that story, Shibata decided her partner "could only" be Mafuyu. You keep wording it like they were pressured into or trying to accomplish something with it, but they easily could have said no, or given her something else. It's what it is: bad writing. It's the direction they took, so why would they make it non-canon? Fan pressure getting them to change it is just wishful thinking.
It isn't something any fan has to accept as being the truth because it's incapable of being the truth and makes it impossible to enjoy the previous entries
You're speaking for yourself here. I enjoy the past games just fine. Was I upset with what they did with Miku? Of course. But taking a step back, I realised it's not my job to try to justify or denounce it - if that's the sort of thing they're going to write and think I'll enjoy, that's on them. If they want that as part of their canon, let 'em. I'll take my coffee scenes and psychic lesbians and be on my way. Will I criticise it whenever I encounter it? Yeah, because it's ridiculous. Do I take it as seriously as I did initially? No, for the same reason.
Ultimately, it being canon has no affect on 1 or 3 for me as it doesn't change anything until 5. Maybe you could try seeing it the same way?
It's completely impossible. There was obviously never an incestuous angle to their relationship
They spun it to be incestuous in 5, which wasn't a plothole-inducing retcon, considering how consistently Mafuyu was one of the only things she cared about. I hate it and don't think it's in character or was ever the case, but I don't think it's "proof" of it being "impossible".
It feels like they didn't want to do this and never have supported actual incest and made sure to show their disdain for it by making it an unpleasant and miserable experience for everyone supposedly involved.
Again, I think you're just seeing what you want to see. A large part of the problem I had with Miku/Mafuyu in 5 is that it's romanticised to hell, and the game seems to want you to think it's all justified in the end, especially with Miu saying she finally "gets" it. You're supposed to find Miku's love for her brother tragically sad, not repulsive or destructive. I'd even argue they wanted you to sympathise with You's love for Sakuya in the end through giving us little glimpses of humanity in his final notes and tape. The difference there is the tone; You is undeniably a bad guy, Sakuya is the creepy 4th floor patient, and the product of their relationship is a 12 year old psychopath. The Hinasakis are portrayed a lot differently.
I don't agree with many things in fiction.
The point is it's not the same thing because if it was I'd be unable to accept those storylines as being canon due to how frustrating they are to go through and how they play with your emotions for the sake of it.
So, you admit that you're defining canon by what it does to you emotionally, not what it actually is? Cool.
Objectively, no matter what, that subplot and any fanfic characters in it will never be canon.
It's been canon since 2014. Please don't drag this out.