I'm starting to think we'll never get to a final agreement, since You seem to put a lot more importance on the personal narrative of the character than I do.
That's because, for me at least, it's exactly that importance of the personal narrative of
my character, and the degree of control I have over that narrative, which makes an ostensibly open-world roleplaying game like Fallout 3 so compelling in the first place. It's
that what occurs in your game can be so drastically different in my game, or not happen at all (and vice versa) which makes this sort of game so special. Anything which diminishes this ability to decide my own narrative arc within the confines of the skeletal structure of the overall narrative, while not necessarily being the end of the world, is still not exactly a step in the right direction, as I see.
Take, for example, Assassin's Creed 2. I thought it was a great and highly-polished game, and even quite immersive in it's own right. But it's a linear narrative.
My Ezio is no different (beyond maybe what colors he dies his clothes and what weapons he uses) than anyone else's Ezio. Personal choice and the ability to dictate the direction of the narrative is not one of that game's selling points (which, of course, doesn't make it "worse," but just a different approach.) That AC2 also features DLC. Now, with that game, I'm sure it's all considered to be "canon." And I see no paradox, there. Because rather than being what I have decided to use or not use in constructing the narrative that is relevant to my experience within the game, it's still a linear story where personal input does not radically alter the direction of the narrative. If I haven't downloaded or played a particular DLC in that game, it's more akin to a series of books and I just haven't read one of them. (ie, it all "still happened," but I just haven't read that one, yet.)
But with something like Fallout 3, I feel that approach goes against the very grain upon which the game is founded. If personal choice is one of the foundations upon which the game is designed - for my character to be as unique, and my experience to be as personal, as possible - then "it still happened even if you haven't experience it yet" diminishes my ability to decide for myself what my experience has been. If anything, we're supposed to be opening up that aspect and expanding upon - to allow the player's experience to be as unique, personal, and customized to their choices as possible.
Mucking around in canon - while not the end of the world - is taking a step backwards from that very concept. When instead we should be pushing the envelope and allowing greater degrees of control over our characters' experience. Nailing elements down as "canon" simply takes away from that choice. Even if objectively we could define canon in this regard, I still don't see how I can subjectively view it as a good thing. (And quite frankly, looking at it from that angle - I don't see how anyone could be particularly happy with a concept that minimizes - or removes entirely - one's own input on what "happens" in their own game.)