Here's where we disagree. A story establishes certain facts as true (Mei and Yuzu are stepsisters; they will get married, etc.). Until such time as a story literally changes our understanding of those facts, they are canonical.
The same is true, albeit perhaps arguably a bit more obliquely, of characterization. Certainly, as I said before, the story can subsequently supplement or transform explanations for a character's behavior or personality, but until it does, anything besides the text itself is rank speculation--Mei might be autistic, or a rape victim, or have suffered brain damage in a fall while staying at fiancé #2's house, or have undergone hypnosis to remove her previous lustful impulses, or be planning yet another arranged marriage to a man. But all of those possibilities amount to projection by readers and are beyond the text.
Okay, I actually agree with you here for the most part. I've been misunderstanding your points up to now.
When I say "there is no canon" what I mean is that readings that can be found within a text and are not contradictory to facts already established within the canon are themselves equal to canon
I believe that Mei can be read as autistic and that this reading doesn't contradict any facts already established within the canon. So, I see the reading of Mei as autistic equal to the reading of her as not autistic. And thus, neither position is canonical. Hence "there is no canon" (or the other way I phrased it "both are canon")
The only part where I seem to disagree is that I don't see these interpretations being "beyond the text" like you do. I see interpretations as a requirement to analysing a text hence why I think that, as long as your reading is based within the text, any given interpretation can be textual.