Eh I had a similar theory a while ago. Only I think it is more likely that humans are the result of spirits trapped on this side of the veil due to the fact that there are implications that the veil may have been constructed in part due to Solas, meaning that Elvhes already existed in some form then.
It's much more likely that the fade itself is a construct. And we're already certain that the fade was a construct (by the elves, because no-one else was around back then who interacted with the fade).
O.o Where did the idea of the Fade being a construct come from? Can't say I've seen that theory before. Veil being a construct, yeah that's extremely likely due to references to a "time before the Veil."
Please explain Abelas. The elves, or rather, the ancient elves did not lose their immortality. New generations of elves did, but the cause is not yet clear. However, elven immortality was not lost by some strange magic, because Solas, Felassan, Abelas and all his compatriots ARE STILL ALIVE.
Elvhen immortality has to have been lost by the older elvhes at some point otherwise the world would still be swarming with immortal Elvhes. I don't think Abelas is immortal, just long lived due to his slumber. Solas however probably has maintained his immortality. The way I imagine things played out is probably something like this:
-Ancient Elvhes form an empire spanning fade and the real world with no Veil to stop them from moving between them. Said Elvhes are immortal due to hearing the ancient song (same one that sustains Cole)
-War between the Elvhen gods occurs, at the end of the war, Solas puts the Veil in place. Has the side effect of cutting Elvhes with weak connections to the Fade off from the Song. Only strong dreamers maintain their connection to the Song
-The leaders of the Elvish Empire, fearing their now mortal life spans turn to blood magic to extend their lives (my understanding is that elves using blood magic to extend their lives was referenced in the Masked Empire). Use of blood magic further weakens their connection to the fade, eventually resulting in dreamers becoming extremely rare.