The Nature of Fade Spirits

Spirit vs. demon might be more of an arbitrary way of defining and categorising them. The reality might be more like Solas suggests, that they are essentially the same, but can change or appear different, depending on how mortals perceive them. It also appears to be possible to corrupt them through abuse or by turning them against their purpose.

Still, it might be true that some of the first spirits to come out of the Void were much more chaotic or primordial, and so they could represent more primal or base emotions, such as Desire, and mortals would interpret these as being demonic influences for this reason. 

It's possible the Chant is correct. We know there was a time before the Veil, and spirits could freely move. But post Veil, they became trapped on one side, and could no longer join the living. Some spirits could have become frustrated and angry, and resentful of mortals as a result.

I have looked at it from the point of view of the old Dalish legend about the origin of the Creators. It seems to be that they were powerful nature spirits that were given form and consciousness when the "Sun" (the Maker?) touched the earth. There was always a Land of Dreams (Fade) and a Waking World (Thedas) but simply not a Veil between them. Spirits could pass over to the Waking World as they wished and could interact with the beings that were there. If they stayed for any length of time, they became more material (like Cole). Some never crossed over at all and had no wish to do so. Those that did had a wish to guide, teach and inspire. All these beings qualified as being First Children of the Maker. The first to arise were those that became known as the elven gods and their primary drive was to be creative, hence the term for them Creators.

However, in the course of creating their beautiful structures they aroused the Titans (primordial beings) whose earthquakes destroyed many of these. So Elgar'nan and Mythal led the others in a war to subdue the Titans. This had the effect of starting to change their nature from simple creators to more complex beings, their purpose being twisted into that of rulers, conquerors and destroyers. I think that this was the war that Solas says started their rise to godhood.

According to the Chant the Maker decided to make things more interesting by introducing humans to the mix. This definitely happened before the Veil was created, the Keep confirms this. The Dalish also claim that coming into contact with humans had an effect on those who mixed a lot with them. I think this was probably true and those spirit beings that did became more mortal and emotional as a result (just like Cole), which was the origin of the idea that humans "polluted" them.

Finally, Solas raised the Veil. Spirits that had once been able to pass freely between the two states of existence could no longer do so. This frustrated them as they could no longer fulfil their purpose of teaching (wisdom) and inspiring, so they became twisted into Pride and Desire, since the only way they could make contact with the world was by responding to mortal emotions and these were the strongest, together with other emotions such as rage and fear. Those spiritual beings trapped on the other side of the Veil in the Waking World either went into Uthenera to retain some semblance of their being or tried to make the best of it by reproducing. Each generation born outside the Fade is further and further removed from what they once were, resulting in the modern elves of today.

I think the Chant is mostly focussed on human history and thus predominantly the time post Veil. I am not sure if Andraste's visions simply did not include some 5000 years of pre-Veil history or whether those who subsequently decided what went into the Chant decided to omit anything not fitting with their idea of what it should contain. There is at least one instance, in what is acknowledged as one of the oldest sections of the Chant, where it refers to the Fade as the "Beyond". This is an elven term, which suggests to me that there might once have been more about the elves that was subsequently "lost". Obviously if Drakon and his tame Divine wanted to discourage interaction with spirits, they would not want anything in the Chant to suggest there was ever a time when this was mutually beneficial.

I think it was not just elvhen that Solas changed for the worst on raising the Veil but he altered the nature of all spirits. It could also be equally disastrous for spirits as for mortals if he simply drops the Veil again.