I wanted to reply in the comments on this post, but my answer got too long. Hope this will be helpful. Thank you @cryptid-catt for the wonderful questions. It was a pleasure to turn all of this over in my mind again <3
I don't think Solas was exactly coerced. On the one side, he was undoubtedly under Mythal's influence for a very long time, both during his life in the Fade and after. However, his pride was constantly pushing back from the other side. In this conflict he could have broken. Leaving the Fade may be a manifestation of his broken will and a step away from his own interests and vision of life.
The name Solas in Elvish means 'to stand tal'. He didn't want to leave his spirit form, but did so under influence, breaking down. So I think Solas may actually be his first name, before the physical form. After leaving the Fade he may have become Fen'Harel, as a reflection of betraying his own principles.
The spirits. Oh yes, they are simply two extremes of one manifestation. Black and white. But we can reflect on this because the lore initially states that some spirits are good, some are evil - demons.
And if we philosophize, then any action can be reframed into grey zone, We could consider that the Fear demon said everything it said from its primary impulses, its light side, in an attempt to remind people of their deepest fear, so that someone like Blackwall, or any other Inquisitor companion in the Fade, would reconsider his entire life and do something good to prevent his fear from coming true.
Same with Solas. His is a very deep character whose views change due to shifts in the world around him and the people in it. So yes, it's likely he is driven by different feelings, including the feelings of various spirits-demons, especially after leaving the Fade, since his personal experience became much richer and his personality underwent many change, but his original disposition as a spirit and its corrupted form dominate at every stage of his life. That is why Solas does everything he does, and then regrets it, but doesn't retreat from his decision.
Mythal is a very important character in his life, and it's very difficult to examine the formation of Solas without her. Because it was she who pulled at him, even without being present in his life. This is easy to notice, given how quickly he abandons his unbreakable plans the moment Mythal lets him go.
Perhaps a spirit retains its true essence only within the Fade. Upon leaving it, a dissolution begins, brought on by the feelings that overwhelm the physical body, for in the waking world everything is subject to desire and passion. This was the case with Cole, though we don't know enough about his future. This was the case with Justice. This was the case with each of the Evanuris, who became the very opposite of what they once were. This was the case with Solas. And I thinl this will be the case with Lucanis's Spite as well, despite the fact that he is a demon.
And perhaps Lavellan's spirit does not undergo such drastic changes and retains its true form. That is why Solas says that he has only ever seen something like this only in the deepest journeys into the Fade. And we all understand what he meant by that.
