See, this is a misunderstanding of that group's purpose. They are there to make sure that story's trend as close to canonically established rules as possible. Why possible? Because writers have the right to override canon based on if they want to make a cool thing happen. Someone earlier made a big stink about how Holdo's suicide run broke canon rules of hyperspace. I don't know or really care about that, but for what my word is worth, if Rian wanted to buck the rules of canon so he could have Holdo's sacrifice happen, that's fine by me.
As for 'coherence', almost every storyteller of any kind strives for coherence, and almost every one of them fail. Coherence is a goal that most writers go in knowing they'll fail in some way. But it's not a part of the definition of canon.
Once again: The literal and singular purpose of canon is give rights to the IP owner so they can legally make money off an IP. Nothing else.
You are confusing the legal right to exploit an IP the way you see fit and the specific meaning of Canon. Yes something being Canon, in a fictional expanded universe (MCU, DCEU. LOTR etc...), means that the content usually originated from the rightful owner of the IP, but it is not the meaning of the term and how it is understood. Canon serves in world building and in storytelling.
"an accepted principle or rule"
https://www.merriam-webster.com/dictionary/canon
It is about different stories and lore elements sharing a same valid, unique and coherent (or which does not fundamentally contradict or undermine the validity of other elements in this shared world) universe. The "Legends" brand of books are NOT Canon, they have no validity related to TLJ for instance, but are still legally owned by Lucasfilm.You have derived products legally owned and distributed by LF/Disney which are NOT canon because they do not possess a storytelling or world building purpose: handbag, T-shirts, promotional material etc...
SW has this strength: the universe of these movies is so iconic and so deeply loved by people that it is a huge undeniable commercial asset. When they release a product marketed as canon, the real value of the material (usually they are really bad on their own, but to each his own) is how it fits in the official SW universe/Canon. The rule is: when we release something we will make sure that it respect the rules and principles we, LF, have already established or defined as Canon.
Sure the Story Group is not perfect, and can let things slip. But these failings can be legitimately discussed, because they can pose issue. Holdo is one of the example: her move is bad-ass, a true highlight in the movie, but the logic of it just break the universe.
In the end they can do whatever they want, they own it. But they have to be honest about it, and stop shutting down people with legitimate criticism. Now Hidalgo and some other people are mocking these criticisms and putting them in the "fan didn't get their wookiepedia movie" basket. The irony is that they will release a bunch junk books, defined it as Canon (in the sense defined earlier) and trash it latter. If you don't care about continuity, or if you want total freedom, don't create a official Canon. Put it in Legends.
What counts force ability and training? TFA and TLJ don't defy canon.
For instance, consider her use of the Jedi mind trick. The movie does not set this up, foreshadow nor justify it. We are showed this, and told to be okay about that. It breaks good storytelling convention (TFA is so edgy...) and also the rules of the SW universe: you got to learn this shit and above all you need to be aware of its existence in the first place. Funny enough JJ Abrams said in an interview she learned it from Kylo's mind. So in the end he use a bad out-of-the-movie excuse to justify a bad deus ex in the movie, and we can be pretty sure it will not be used again because of it.
This example is funny, imo, because I am pretty sure that if JJ's "explanation" had been established in SW Canon before or after the movie, you could have gotten away with it, kinda. The lore, if you use it well, can help you out (it would have hurt less). It is a pre-existing set up that can save your ass if you are a bad writer.