This is a really interesting theory, and does solve the question of why Solas would not be suspicious of something that looks like a walking skeleton. [Or if not a skeleton, obviously infected with the blight.]
I wonder what could have made Solas fall back into a slumber?
er, well. If you subscribe to Madrar's Crazy House of Theories, he doesn't.
1. After the magister Sethius Amladaris betrays him, taking the orb that represents most of Solas' remaining power, unleashing the Blight and adopting the name "Corypheus" to celebrate his power over the corrupted song, Solas basically does what he decribes his plans would have been if you'd been unable to close the breach: he high-tails it out of the path of destruction and keeps a low profile, all the while trying to figure out what's gone wrong and what he can do about it.
2. The answer is "not much" for quite a while. At some point, I think he discovers that the spirit of Mythal has managed to claw her way into a mortal body. They collaborate about what can be done. The Andrastean slave rebellion ensues.
3. Once again, things do not go entirely to plan, but there's something salvageable in the formation of the Chantry: an organization that does not seek to govern people directly, but rather provide a moral compass and some pointed warnings about letting mages have all the power.
4. This backfires spectacularly, thanks to various historical events and political machinations, twisting its original message to one of distrusting elves and suppressing mages. (Bonus: the painfully literal interpretation of restoring a time when 'everything sang the same' as spreading the Chant to all corners of the world.)
5. Solas continues to try to track down what happened to Corypheus and his orb. No luck. Still pretty much powerless, he spends more and more time looking for answers (or just the comfort of familiar spirits) in the fade. Flemythal retreats to her swamp.
6. Events of DA:2. Corypheus is back on the scene, and (somehow) Solas catches wind of it. Time to track him down and try to fix the world. Again.