We're Big Damn Theorists
Starting with the Magisters and Lyrium.
From What we know about Lyrium, It can mutate mages who use it extensively. So, we know that the magisters also used very,very large amounts of lyrium when they preformed the ritual to breach the fade. I personally do not think they manage to make it to the fade, but instead it was the earthly equivalent that possibly lays at the center of the world. The use of the large amount of lyrium is what could have created the first Darkspawn, by mutating the mages. An the reason they did not reach the fade is because they lacked the old blood of the Calenhad line.
The Taint. The Calling. The Cure
I think the taint is caused from the tevinter mages getting their ritual wrong in some way. The calling might have something to do with the call that brought the dragons to the temple in the silent grove, before they went extinct. I am almost positive that the cure has something to do with the Calenhad line. Fiona was cured and though she seems unsure how, I think it has something to do with carrying the child of Maric. As we know the possibility of a Grey Warden reproducing is extremely unlikely, but Fiona managed to get preggers, then is mysteriously cured.
The Old Gods.
They are not gods at all. They are in fact Great Dragons. We know of one Great Dragon that is already awake, The Queen of the Dragons (Possibly Flemythal), who was awakened by the blood of Calenhad. Calenhad supposedly drank the blood of a dying Great Dragon in a cave, if you go by Qunari belief of the tale. It is also said by Aurelian Titus that the tevinter mages were close to reclaiming the power of the dreamers (Who could shape the fade), but all they needed was dragon blood. I am still trying to find the attachment of the dragons and the elven gods. I am not sure as to why they seem to parallel each other so much. Any thoughts?
The Dreamers and gods
I do not believe that all of the ancient elven gods were dreamers. There were considered to be 7 sleeping dragons. I personally believe there are only seven true elven gods. Elger'nan, Fen'heral, Mythal, Dirthamen/Falon 'din (Same person), Andruil, Sylaise, and June. I do not think Ghilan'nain was really a god, maybe someone who was highly praised and later was attached to the gods.
The Veil and The fade.
The Fade did not always exist as we know it. I think the veil was created using blood magic of some kind, perhaps related to the great dragons. The reason I think it was created by blood magic is because it is not a true physical barrier, but one of the mind.
Solas
So, I think I have an idea of Solas's agenda. In a conversation he has with the inquisitor (I think a rival relationship) She approaches him about not caring for the elves. The quizzy can ask if he can think of know way, and he states not unless we collapse the veil and casually reshape reality. I think this is possibly his true plan. I know you have a quest to strengthen the veil for him, but perhaps he knows of a way to bring it down that is not as destructive.