Rivaini religion may be minor in its presence on Thedas, but there's too much emphasis on the idea of the world itself being alive to discount it.

Well see, I'm not entirely certain about that. If we look at it in a certain way, we could say our own world is alive, with what we know about volcanoes, organic matter, carbon and hydrogen, right down to just talking about atoms, and so on and so forth. In Thedas we have lyrium being described as alive, dragon blood being the blood of the world... but none of those things are fully understood, by us or by the people of Thedas.

There's an interesting and utterly inconsequential moment in Skyhold with the surgeon, where she talks about focusing on physical medicine, "a balance of the humours," instead of relying on magic for healing. I found this fascinating: it's evidence of an emerging interest in science, of trying to use other paradigms to explain physical (and sometimes metaphysical) phenomena. Obviously we can't directly correlate our own world to Thedas, because we don't have magic, but in a broader sense when looking at how both worlds have throughout history looked to the supernatural or non-scientific things to explain events and solve problems, both worlds have come up with their own justifications for those practices, that were perfectly understandable and seemed reasonable at the time. But we know now that, for example, the world isn't carried on the back of a turtle. Similarly, the way that we, and the people of Thedas, might interpret what it means when it's suggested that the world is alive might change when we are given more information. There might, at the heart of it, be a perfectly rational (to our understanding) way to explain what that means. There might be a perfectly scientific way to explain how lyrium works as an organic-mineral hybrid. There might be a scientific basis to explaining how magic works in their universe. The problem is that we don't have the knowledge yet, just as in the 1400s people here didn't have the knowledge to explain a whole host of things we take for granted today.

So it's interesting (and understandable in many ways) that we - as modern, scientifically-literate people - so easily revert back to a more primitive mode of thinking when presented with a world that hasn't learned what we have learned about our own yet. It doesn't matter that their world isn't the same as ours, because what I'm getting at is methods of thinking, rather than the details of what we're thinking about, iyswim.

But, to get back to your point about the Rivaini, I don't see an atheistic approach and their Universe/Natural Order approach as necessarily being mutually exclusive, since the end point for their belief system could well lie in scientific understanding. Not that I'm planting my flag as Thedas' biggest atheist, and suggesting that there is no supernatural Creator or anything else; just that with the information we have right now, the ways different cultures interpret the world around them don't necessarily invalidate the possibility that there isn't any kind of creator beyond the processes of nature and 'science'.

(I'm totally on board with the idea of there being a god or gods, it's just that I interrogate it and come at it from the default position of wanting to focus on what we know is 'real' rather than assuming there is truth to legend and myth.)

But, overall I like your line of thinking about their culture being the least likely to have been 'diluted' or 'corrupted' over time. I think it would be fruitful to create some kind of post, a timeline, that chronicled the different beliefs of different cultures, and at what points they have altered, diverged, become more similar, etc. I will say though, even if this ancient knowledge was passed down directly by Rivaini witches/seers, we are still left with two problems: 1) we have no way of knowing what influence the events of the time would have had on the way each individual seer passed down that knowledge, in other words it's still open to change and interpretation over generations; 2) it assumes the further we go back in history the more likely the beliefs held there are to be fact, which isn't true.

On that second point: societies move slowly toward truth, as they find more ways to accurately interpret and explain the world around them. There's a presumption in our readings of Thedas' lore that the more you can uncover of the earlier tribes and peoples the closer we'll get to discovering the truth of creation, but in fact there's no reason to believe that the very first people who 'invented' a particular belief system did so as a result of incontrovertible fact that has since somehow been 'lost' to time. There's nothing to suggest that the very first people to come up with the idea of a 'Maker' (under whatever name it may have had) had firsthand experience of the actual events they are attributing to this creator, and yet that's something we keep presuming when we dig further and further back in our attempt to 'get to the truth'. Likewise with the Rivaini, there's no reason to assume that the people responsible for setting forth their belief system ever had indisputable evidence for the truth of it.

In reaching as far back as we can in the timeline of a particular belief system, all we might find is the first person who sat down one day and said to themselves, "I wonder why we exist? Here's a theory..." All we can do, as lore-hunters, is focus on the events and people we have proof for having existed. For example, we know now that Mythal existed, but there's still a lot about her that we can't pin down to distinguish between fact and fiction, and we may never be able to. Likewise, we might be able to explain one day what it means for lyrium to be alive, but it might not lead us to a place that lines up with various belief systems of Thedas.