:facepalm: :facepalm: :facepalm:
An in-universe theory without a credible alternative or reasoned doubt is an in-universe fact. That is how we know that Ysgramor did not make landfall in Newfoundland after all.
Actually, "theories" and "facts" are not the same thing under any circumstances. Theories are supported by reasoning based upon facts (or, at least, good ones are), but theories themselves are not facts.
For example, the statement that the Dwarves disappeared around a particular date is a factual statement. Even if I said they disappeared just last year, that would be a factual statement... albeit an untrue one. Saying that Nerevar used the tools to make them disappear (Five Songs, IIRC) is a factual statement. So is the statement that it was Kragenac that used them and made them disappear. Which of these statements is TRUE comes down to anolysis of corroborating evidence, making a decision as to which is true a matter of opinion, not fact. Hence the controversy.
Saying that the Tribunal used the Tools to gain their power is also a factual statement. Whether or not this was a good thing is a matter of opinion.
As to matters astrophysical, it seems to me there is very little reliable evidence
Yes, it is fact. Lore is literature and not science. The sole window into the world of fact and falsehood is the one provided by the author.
The problem with this statement is that the literature present in the Elder Scrolls universe is not present as absolutely factual in nature, unlike other games. The in-game literature is mutually contradictory, and at times contradicts even with in-game observations, sometimes deliberately (the Amulet of Kings, for one), sometimes not. If, for example, the devs, for TESV, decided to create an even more realistic sky model, with celestial bodies behaving in a manor contrary to the Temple Zero model... what would we do? One response would be the "scientific" response, of assuming the Temple Zero model was wrong and whatever model the new in-game was the actual truth. The other would be the "Scholastic" response, by which the developers are declared to be morons.
Of course, the problem with Crimson Paladin's assertion is that, so far as I can tell, there is no evidence for alternative models. He speculates that the Dwarves may have had the ability to gather such evidence, but so far as we know, Dwemer observations, while lost for the most part (with the exception of what can be gleaned from Dwemer Orreries), may have formed part of the foundation of what is passed down via scholars. We have no way of knowing, one way or the other.
However, I think the heat of this discussion stems from a perception that there are individuals here who will go into flame mode to defend certain single-source "facts," and that there is a particular body of sources that is erroneously regarded as significantly more credible than others.