Well....the attitude you are expressing here makes me a bit uncomfortable, even though I agree that there is no problem with ESO.
You can't just change whatever whenever. Or you can, but you shouldn't because you will destroy your world and your fan base.
There are changes that makes sense and there are changes that don't make sense. Fleshing out a previously unexplained part of history makes total sense and there is no contradiction. Other changes, like if they change Moon Sugar from a hard core drug to a benign substance, make absolutely no sense at all. The rules of the world shouldn't change. Biology shouldn't change. Those types of things ought to be set in stone. I think its a serious problem when people assume that lore is just history. History is one small part of lore, but there are lots of other things that go into it. The lore is the entire foundation of the world and it needs to remain consistent with itself in order for it to be believable and thus immersive. I'd encourage you to read Tolkien's thoughts on the subject. Google "Tolkien second world" and I'm sure you'll find it.
Wrong. The Dark Ages were very much dark. We now know a lot about that period of time but only because our epidemiological techniques, ie. our ways of gaining knowledge, have improved so much.
Common people in particular knew very little at the time and immediately following that time. That's a fact and the only thing that is relevant to our conversation here. The vast majority of knowledge was maintained either by priests of the Catholic church or by other cultures. Literacy rates where at their lowest point in all of history since the invention and spread of written language.
Now we know a lot more because of techniques like archaeology and because there has been enough research to gather all of the then disparate sources together. Still though, there are things that were lost that will never be recovered (look up the Burning of the Library at Alexandria for example.)
What we know now though is completely irrelevant to the conversation. Skyrim in the fourth era is anologous to the mid to late Medieval period at best, not the freakin 21st century.