The bottom line, in my humble opinion, is that "canon" is the lore than makes it into official media like games or books.
Now, that point might seem a little redundent in light of the conversation at hand - however, I think it still bears repetition. MK may be a valuable contributer to TES lore, but if his work doesn't make it into some sort of official media, how could it possibly be considered canon? Some of it makes it in (Knights of the Nine lore), and some of it doesn't (potentially, the Loveletter). My point is, MK's position as a key lore writer doesn't mean that all of his work becomes canon. That's the summation of this argument, right?
You are correct in a very technical sense. Whatever does not appear in official games or official books/media is not official. But the true point here is that officiality in this sense is virtually irrelevant in terms of canonicity.
Once again, I bring up the case of the http://www.imperial-library.info/obscure_text/census_daedra.shtml This was written for the Pocket Guide to the Empire v3 that came with collector's editions of Oblivion, yet it was cut due to length constraints, not due to content. So it is now an obscure text and is technically unofficial because it does not appear within the games or within official media. But does that make it truly any less accurate or canon?
Or what about little things like http://www.imperial-library.info/obscure_text/cosades_visit.shtml? Ken Rolston, lead designer of Oblivion, posted that, and is that now inaccurate and untrustworthy simply because it doesn't appear within a game or an official medium? It's not hugely affecting anything; it's not quite of the magnitude of the Loveletter. It's simply a servant of the Empire paying his respects, which doesn't contradict anything official. And it was, again, posted by Oblivion's lead designer.
And the final point is that most of the core information that is extracted out of MK's obscure-text work can also be extracted out of his in-game work with a bit more effort. http://www.imperial-library.info/obscure_text/many-headed_talos.shtml as compared to http://www.imperial-library.info/obbooks/mythic_dawn_commentaries.shtml, as an example.