But really, when you think about it, fans shouldn't even worry about what is canon. The only people that really should be concerned with canon are show runners because they are creating it (and even in that case as Christopher noted, they are not beholden to it---though it's probably a good idea for any show runner to be familiar with the canon). And more importantly tie in writers like authors and comic book writers because they have to be consistent with the canon. And that's it.
You're sort of contradicting yourself here, because what you're talking about is
continuity, and you just said that's not the same thing as canon.
Creators don't have to "worry" about canon one tiny bit, because whatever they create is
automatically the canon. "Canon" is a word that only has meaning in relation to the stuff outside the core work. If there's only the core work and no tie-ins, the question doesn't have to be asked, at least not unless later installments choose to ignore earlier installments, like in
Dallas or the
Highlander or
Terminator film series. Generally it's a question that only applies to the tie-ins -- are they considered part of the canon or not? That's a tautological question where the canon itself is concerned, so the creators of the canon never need to ask it,
unless they choose to concern themselves with the tie-ins, or unless they actively choose to remove some past piece of the canon from continuity.
Asking "Is this canon?" is kind of like asking "Are you on Earth?" People who live on Earth don't need to wonder if they're on Earth. People who create a canon don't need to wonder if what they create is canon. It only comes up once you move outside of the thing in question. Otherwise it doesn't even need to be thought about.
So for the question I would say for now it's probably safe to assume the prequel comics and novel coming out are probably part of the continuity. But as time passes it seems more likely they might contradict something in that extra universe stuff. They may not, but I think it's clear the current show runners, while they seem to want things to be tighter within the shows and their tie ins, they don't feel constrained by any of the tie ins if the story they want to tell necessitates that. Then it's up to future authors to try to reconcile that if they can and want to. Sometimes inconsistencies can be hand waved, and sometimes creative storytelling can reconcile stories, and sometimes you just can't and you have to let it go.
I've never understood this desire for some kind of official permission to count tie-ins or not. I've always seen that as my own choice. If I think a story is compatible with the screen continuity, I include it in my personal model of the universe. If a new screen story then contradicts it, I remove it from my model. I've been reworking my personal Trek continuity for decades, constantly adjusting it, taking tie-ins out or putting them back in -- sometimes taking
episodes out or putting them back in, depending on whether I felt their ideas were too problematical to reconcile or not. I mean, why should I need any official permission for how I choose to see things inside my own head? As a professional author under contract, I'm obligated to follow CBS's lead, but when I'm just a fan using my own imagination, I can do whatever I want.