I think there can be multiple canons! What
is a canon? It's a self-consistent set of works, intended by the creator to fit together. I would usually add "complete enough to reasonably write fanfic of", because that's how I think.
So Tolkien published a set of books: LotR, Hobbit,
Adventures of Tom Bombadil. That's a canon. Christopher added to that, with the Silm, UT, and maybe
Children of Hurin. That's another canon, an extension of the first, and (aside from Galadriel and Celeborn) internally consistent.
There's at least two versions of "the canon" which build off that: one which tries to reconstruct the absolute latest ideas Tolkien had, and one which tries to work in his latest ideas
that fit the published Silm. Both are very individualised; no two people are going to agree on exactly what's in each.
But there are other canons! The Book of Lost Tales is a nearly-complete, mostly-internally-consistent canon.
The Lost Road presents a different canon for Numenor, with different timelines and characters, complete enough that you could easily write fanfic in it.
And yes, there are the movie canons. The Jacksonverse, which may or may not include
Rings of Power. The Rankin-Bass-verse, which may or may not include the Bakshi movie. Heck,
the full script of the Boorman movie is available; that's a canon, too.
From a fanfic perspective, and even from a discussion perspective, all you have to do is specify which canon you're thinking of. "This is a Movieverse fanfic, including
Rings of Power." "In the
Lost Road era, was Elendil an effective leader?" "If Gil-Galad is Fingon's son, why did Turgon become High King?" Just try not to cross your canons over without realising you're doing it: "I think the Stranger is Fionwe, son of Manwe!"
The difficulty, of course, lies in those two individualised canons: the ones that ask what 'really happened'. For fanfic, you can cite the specific text - "I'm going with the version where Galadriel meets Celeborn in Aman". For discussions... well, that's why their discussions: you bring your preferred "canon" texts and make a case for why they're the most like what 'really happened'. You can forge agreement, but never a definitive answer.
hS