Building a library...
I'll make the answer almost as short as the question : libraries have books, SE have Q/A.
...of canonical questions?
here, "canonical" sounds like (kind of) "rhetorical". And that's not (at all) the SE/IPS format.
I realize that it's a bit harsh. Maybe it is. Too much. And if it is, I'll apologize. But that's exactly the short answer that I think needs to be done sometimes, like when we say that "NO is a complete sentence". And that not too many explanations shouldn't be given. Here, though, we should provide people with explanations, so, here are my 3 cents, as you have 3 bullets in your question.
What format do we want to use for those canonical questions? -> none? (because it doesn't fit our SE format? Op-Based / Too-B. / Off-T. / ...).
What kind of question do we want? -> the one we can answer, because they have a real problem, a real goal, an IPS solution, and they respect the SE format, I'd say.
Are they some that we don't want? -> all of the others? ;)
Short story long now, just from some of your examples:
How to support my sexually assaulted friend? -> it depends on the friend/age/perpetrator/so-many-factors-you-can-name-and-add... it's far too broad, and words and actions will vary depending on the relationship(s), the personality (both you and your friend)... and so on...
How to know if you are in an abusive relationship? -> was already discussed on meta.
How to help my depressed/suicidal friend? -> was already discussed on meta and an answer was given, so that one can copy/paste just in case.
And just because @JAD already used (in comments) the words and sentences I was about to write, no need to lose time, I'll just quote them, then:
What would make questions off-topic for IPS suitable on meta? Either their format is problematic, which makes them unanswerable on main, I don't see how moving them to meta fixes that.
I think we need to be careful with treating meta as a catch-all for off-topic questions on main. Just because a question deserves an answer, doesn't mean that IPS (main or meta) is the place to put those" (end of quote - My thought: even with links, we're not a "regular/normal" forum).
Judging from the meta about that question, it was both off-topic and too broad. I'm not sure if moving the question to meta solves either of them really. It sounds a bit to me like an answer in search of a question, and if all else fails, just put that question on meta.