I feel you have misunderstood my meaning, and so I'll try to present it again, in a more verbose form.
The function of canon is for authors to create a seamless setting, and that setting's function is to act as the basic point from which negotiations of play contract in your group begin. As such, canon has no role at your table. A person who aregues from canon, as presented in your original quote, is misunderstanding what the canon is for.
This is not the way things work in games where the rules are designed to enforce fairness. D&D for example, uses the rules to blanace play. Ars doesn't do this: the rules are deliberately designed so that your group can do things which, to other groups, would appear obviously broken. So, in D&D if two players say,
"I'd like to play a demon-blooded character."
"I think they aren't any fun and would prefer you didn't."
then in that situation
"But it's in the book!"
acts as a sort of tiebreaker.
In Ars, the conversation should go something like this:
"I'd like to play a demon blooded knight."
"I think they aren't any fun at all and would prefer you didn't."
"It's in Realms of Power : Infernal!"
"And...?"
That something is in the book just means its in the basic state from which negotiation ensues, it doesn't make its presence in the setting better or righter, or give it any real weight compared to "I won't enjoy that. Let's talk some more."
The function of canon is to keep the -authors- in line, so that your initial gamestate for negotiation is pretty straightforward. It's not designed for players to use it to debate with each other. That's simply not what it is for. A player saying "It's in the book!" is actually saying "It's up for negotiation!" and that's generally not what they mean. They generally mean "In non-storytelling games, this has been a trump card for me! Back down!" When actually saying "X is in the book" has no particular relevance to your game state. When my wife sits down to paint, some of her canvasses come from the store with a layer of white undercoat. That doesn't mean the finished painting needs to be white. You have canon as the starting point for your discussion: it's not mean to be used in a "pure" or "by the book" form, any more than the prewhitened canvas is meant to be hung on the wall as a finished piece.
Ars is a game of continual negotiation, at your table, about how you, perosonally, want to play. Canon's not designed to be used as more than a starting point in this. Canon binds authors: not players.A player arguing from canon is not using it for what it is intended to be used for.
Just consider the enormous effort ArM5 has made in the beginning to have rules and the game world, Mythic Europe and the Order of Hermes, match. This was a feat that set apart ArM5 from most other RPGs and defined it over many books. So it is natural for gamers to expect further ArM5 books to follow in the footsteps of their predecessors, and to try to use the rules in them to look for the possibilities and limitations of the game world, Nothing else is ArM5 game groups discussing canon.
Oh, the -books- do, because the books are written by authors, and canon is for authors. Your -game- does not use canon in this way.
I would never tell gamers who expect that from ArM5 off by just stating "Canon's for -authors-". For a few books still, they will - quite rightfully - expect the consideration which the core book, the HoHs, A&A, C&G, LoM, The Church and also MoH have shown. I have given a few examples of this already, and here would refer you only to David Chart's design notes on the Atlas page introducing ArM5.
I'm not clear on how the documents you cite support your view. Indeed, I think they say the opposite pretty clearly.
Of course, Apprentices is not to be used that way, and maybe the books following it are neither, so we have to break it to our groups that these books are mere quarries for ideas, as typical RPG supplements are. To do this, Matt's candid post in this thread will help a lot, as will David's explanation, how difficult it was to complete Apprentices.
Apprentices is canon, in the sense that when a future author decides to stat an apprentice, he or she will be expected to follow these rules or have a good reason why they didn't. If, thought, one of your players is using "It's written down! It's canon!" in a discussion around your gaming table, then they have misunderstood what canon is for.
I feel, that once an RPG has a certain amount of material assembled, writing further quality stuff becomes more and more difficult, as the initially necessary shortcuts start to catch up with you. And if you then attempt to write on a subject like apprentices, which lies at the root of almost each of the defining PCs of the ArM world, this problem is likely very much exacerbated.
So it is logical to find Apprentices as a watershed book, in which it has become very clear that from now on the Troupe needs to march on its own (see the French childrens' song 'Dans la troupe': "La meilleure façon d' marcher, c'est encore la nôtre / C'est de mettre un pied d'vant l'autre / Et d' recommencer."
), and looking for canon in the rules will not sustain it any more.
Cheers
I'm not able to understand the French song, I'm sorry so I hope it didn;t make a particular point by it, but even in this last sentence you seem to be suggesting that groups could previously play in a sort of "pure" way advocated by some D&D players, where it's all by the book and only by the book. Ars has never worked that way. Never. It has always been about your choices, expressed through your play contract. That's why when there are disputes we say "Ask your troupe" not "Ask your GM". The rules have never been the one proper way to play, they've always been a guide to negotiation of play.