More like I'm not substituting everything with crackpot theories from secret societies in the game.
Thing is, it's just about the only thing that currently exists that attempts to explain that part of lore. It's all we have to go on. Of course, like with all the conflicting creation myths each race has and how you must view the true story because of that, it would seem wise to take the cosmology text in the same manner. Useful information, but probably less so when only taken at face value. But the lack of other accounts in this case makes that a bit trickier than figuring out creation.
A big part of the cosmology text is to explain how the quirky things we seen in TES really work. Have you noticed that stars can be seen through the moons when they are not full? How can you possibly explain that through conventional cosmology? Nothing in TES really meshes with our real world, unless you are willing to dismiss magic and myth as real forces (which I'm going to guess you're more than ready to do).
Oh, and also how the unlit sections of the moons are very noticeably darker than the "atmosphere"'s (atmosphere anolog's, whatever). I do remember seeing the same effect with Earth's moon as well, but to a
far lesser extent, but maybe that's because I'm just getting tricked by my narrow retinas and all the palinopsia and whatnot I get with that.
Also, going back to your bit where you stated that what goes on in Nirn is only a small fraction of the entire picture... again, I'd be very hesitant to say that. First, we've only really seen Tamrielic lore, aside from a couple things such as the Tsaesci (I may have spelled that wrong, I'm tired) creation myth. Well, that and we've only really seen at all is Tamriel. Nirn encompasses areas outside of Tamriel, of course; of these, we know of Pyandonea and Akavir, and possibly Atmora and Aldmeris, though their existence is suspect, at least the latter's is (I can't remember for sure if there was anything calling Atmora's existence into question, but it seems like Man's counterpart to Aldmeris/Old Ehlnofey, so maybe?). Point being, there is for sure a lot of important stuff going on in Tamriel alone, regardless of how proportional these events are to things occurring elsewhere. And there's other lands on Nirn that I'd suspect also have a lot of things going on. Taking Nirn as a whole, I'd say it's pretty significant, don't you think? Though I'll admit there's also evidence that would make it less so. We have this, from that text I linked to:
Dyus:
The limited imaginations of Mundus’ inhabitants are very amusing. There are as many types of servants as stars in the sky. Daedra, as you call them, are but one of the more useful. The Knights of Order are yet another. My Lord Jyggalag is partial to their lack of creativity and independence.
This states that there's all kinds of other beings mortals on Tamriel have yet to encounter. These beings are also labeled as "servants", so it follows they serve
something, and to serve means action, which means more events. Events happening in places that we haven't discovered, or just lots more events we're unaware of happening in places that we know about. That could marginalize Nirn's significance a bit, unless a lot of these things are happening on Nirn but outside of Tamriel for one reason or another, but could also marginalize the significance of the Outer Realms we know of. Oh yes, I just remembered,
Haskill:
How typical. You mortals love to take a pebble of information and construct entire realms of conjecture upon it. Your lore of those disparate beings that you lump together as "daedra" is based on nothing -- lies and half-truths told by traitors, rebels, miscontents and weaklings who have had the misfortune to become involved with mortals. Or with Mehrunes Dagon. Do not speak his name to me again. The Master of Scum. The pawn of every Prince of true power, the dupe of every schemer in the Nineteen Voids. Do you think you know anything of the politics, factions, feuds, vendettas, wars of Oblivion? Do you think Oblivion such a simple place, that the tale of the loyalties of a great people such as the Mazken could be encompassed in a brief tale?
Ah, excuse me. I seem to have become rather ... excessive. I am usually better at controlling myself. Forgive me. Pray continue with this most enthralling conversation.
Nineteen voids. The mortals inhabiting Tamriel were only aware of sixteen, weren't they?
Though the inhabitants of Tamriel would tell you that the Daedra are an untrustworthy source of information. Maybe, maybe not, maybe only occasionally; difficult to say for certain. Either way, it's apparent to me that the writers of the lore are trying to paint themselves in a position such that they're not constrained to just what we've seen or heard of thus far in the series, in fact, expanding their potential horizons a great deal. Still, it isn't fair to say that the happenings on Nirn are comparable to a speck when you're only considering Tamriel.