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The sad truth is that there is no way to truly "determine" anything, as it hinges so much on our personal preferences.
Either we like what the other said (and then proceed to adopt it), or we don't (and continue to stick to our initial vision). There just is no one singular truth when it comes to this level of detail.

By their very nature, the "established facts" is the stuff that does not (or should not) differ from novel to novel or from novel to Codex, and the amount of these facts is probably quite small and does not extend to the exact minutiae of the Arbites' (or the Ministorum's) jurisdiction. The only established fact for *them* would be that they exist at all, I guess.
Yes, I'm pessimistic when it comes to cross-medium consistency in 40k.

The trouble with those "established facts" is that they themselves are not consistent in terms of the "level" on which they operate. The idea that "Blood Angels armor is red" is an extremely specific detail relative to the setting -- as opposed to something that is very high level, like "Karl Franz is the current emperor" (keep in mind that WHFB is a much narrower "space"). The idea that Blood angels wear red armor is trivial compared to something as definitive as so-and-so is in charge of a huge portion of the setting of our story.

Meanwhile, we're missing a lot of other high-level details, such as how the Lex Imperialis interfaces with the doctrine of the Ecclesiarchy. (Or even what that doctrine is in the first place!) And how does "Commissar Yarrick defended Hades Hive during the Second Armageddon War" stand up against "Ollanius Pius stepped between the Emperor and Horus on the battle barge"? They're the same level of information (the relevance of the individual to some larger issue) and yet one is considered an "established fact" whereas the other one might be some kind of legend or vision.

So these "facts" have to be considered for what they are: elements of brand identity -- AND NOT -- hard-and-fast milestones in an otherwise ill-defined fictional universe. The goal has never been about singular truth; we don't even have the granularity of detail to start establishing that. The goal is more about making sense out of what we've been sold, which includes video games and novels as well as miniatures rule books. This is what I was trying to emphasize to Aaron Dembski-Bowden in the face of his rather starry-eyed rehabilitation of GW's IP strategy:

Fluff discussions are how fans transform the bitter experience of being (ineffectually) marketed to into the meaningful experience of exploration and enjoyment.

I guess the level of what can be considered an "established fact" is fluctuating, and might even be up to personal interpretation as well. This could explain why some novels get through Black Library's editing process containing things that make a lot of fluff experts go into ragemode. You just have to drop the name "Goto" somewhere and you'll have at least one poster reacting to it with scorn.

On the other hand, I don't think that the examples of Yarrick and Ollanius Pious are actually counted amongst the "established facts". Events like these can be retconned from one edition to another, as we have seen. Perhaps even the idea of Blood Angels armour colour isn't an established fact, for look what GW did to the Night Lords and their lightning. None of this is what truly defines 40k as a setting, what gives it that special "look and feel". It's just ... accessories?
In my opinion that doesn't do the setting justice, but seeing how the fluff is implemented, used and disregarded, that's just what it is. And interestingly enough, many of the authors themselves (not all, as I had learned) prefer it that way. *shrugs*

I also agree about the assessment of the nature of fluff discussions, by the way. That said, in some cases it really is because someone just didn't know / didn't read of a specific source. Such common ground is unfortunately very rare, however, since our interpretations are mostly defined by multiple sources. Sources which, more often than not, conflict with one another, leading to the effects we both witnessed.
That said, I have a feeling that many fluff discussions would be less hostile and more constructive if everyone would once and for all acknowledge that 40k really does not have a singular canon but just a lot of interconnected interpretations, where it's left to us to which one we wish to subscribe to. Perhaps we all need a disclaimer in our signature that says "the above post is based on the following sources". There can still be disagreement, but it shouldn't escalate beyond anything like a "well I think your sources suck", because that's all there is to it - personal preference.

Remember when Dark Angels used to all wear black armor?

As to the hostility, I think it makes more sense when you figure out what people are actually trying to do: having been failed by GW in terms of creating a meaningfully coherent world, the fans are working with each other to make one. Yes, we work together poorly. But when you consider that no one is paid for this, it's still pretty impressive.

Well, more often than not it feels more like very enthusiastic (read: borderline-zealous) fans just trying to preserve their own idea and prove the other one "wrong". Way too many people still operate on the belief that "it's all canon".

Not saying that I myself may not be borderline-zealous as well at times when it comes to my own interpretation, mind you.