Re: Terms
I think we have a good list to work with, insofar that there are Actually Kind Of Too Many words that we can use to categorize and delineate differing degrees of canonicity. So I'm going to Make An Attempt to pair down to four: Canon, non-canon, dubiously canon, and Post-Canon.
Personally, the way I see it, the only work that can be called capital 'C' Canon is Homestuck proper, as without it none of this other shit would even exist probably. To me, that makes it the authority from which other definitions must be subsequently derived. So, Canon is the Text of Homestuck, from page 1 to page 8130 (excluding the credits, but I'll get to that).
The Credits, are not Canon, but they are not dubiously related to Canon, as they technically exist within the text itself, therefore putting them firmly in the Non-canon Category. A category in which I would also place various japery like the First trickster mode Minigame (can it be called that? In either case, it's on page 253) as well as the SBAHJ Comics.
Ergo as The Epilogues, Snapchats, HS^2, and HS:BC, are neither Canon or Non-canon, they are apocrypha(thanks again, @Victoria) of dubious relation/relevance to Canon (Per the dictionary definition of the term). You don't actually have to consume them to properly engage with Homestuck's Canon at all, and the actual Canon isn't influenced by their existence. This makes them Literally Not Canon and they are —as we've established— of dubious relevance, I would call them dubiously canon.
Post-canon is a more vague category, I think. I would argue that most fanworks engage with Homestuck in this post-canon manner, and that subsequently anything that is not either Canon, or non-canon per the prior definition, falls into this category. Including that which can be considered Dubiously Canon; Not all Post-canon works are Dub-Canon, But all Dub-Canon works are Post-canon(Squares and rectangles, y'know). All of these Post-canon works are influenced Metatextually by Homestuck, and are in conversation with it in a roundabout sense, some more closely than others.
And so, according to this logic, our glossary of terms would be as follows:
Canon: The entire unabridged text of Homestuck.
Non-canon: Directly tied to the original Canon, but not integral to the narrative or metanarrative.
Dubiously Canon: Indirectly or Questionably related to, but not a part of, Canon.
Post-Canon: Anything in Metatextual Conversation with the Canon. or, Per @Victoria "an approach to storytelling that rejects a single canonical truth, opening up contradictory ideas, to convey its story (think post-rock, post-structuralism)"
I think that Sorta Makes Sense. Sure.
"Wednesday's Child is full of Woe."