Not sure if this idea has been posted yet (61 pages is a lot to read), but it just occurred to me.
It would be interesting if the Elder One was one of the Tevinter Mages who got left behind in the Fade, wandering for centuries in the land of dreams. Perhaps he even exiled his brethren from the Golden City, bestowing the curse of the Blight upon them and Thedas, and spending the intervening ages unraveling and mastering the magical secrets held in the Maker's own palace.
Probably not the case, but I think it would make for an interesting villain nonetheless.
This would either confirm the Maker's existence and assholery, or it would not explain where he got the Dark spawn taint. Both of those would be disappointing in my opinion.
Him being just a Pride Demon doesn't necessarily take much away from how much of a Pride Demon he can be. The Architect and Corypheus are just Darkspawn. Special Darkspawn, but still just Darkspawn.
No, they aren't 'just' darkspawn. Are archdemons 'just' darkspawn? No, and the architect and the conductor are both even more than that. They are the original darkspawn, who are sentient and have a soul and can hop bodies when needed.
Which actually begs the question, where did the other darkspawn come from? The original magisterial don't really seem all that subservient to the song, and even if they were, they would be digging, not making more of themselves. So what happened for them to suddenly make more darkspawn? How did they do it and why? Did blood magic no longer work as they expected? Or were they under the archdemons influence and did he (instinctively) know how to make more darkspawn? Or was it the Elder One, in control from the Black City through the tear made by the magisters? A tear that clearly closed sometime after that.
The question then would be, why has he been silent for a millennium, why can he suddenly influence the world again? What happened for this to be possible?
Also, if the story of Flemeth summoning a demon is true, it seems most likely that she is bonded with the Formless One, not that the Elder One is him. This would certainly make Flemeth more than a demon, without making her an Elven god. It would, however, not account for her fondness of the elves.
As for spirits and demons, there is no difference between the two. They both embody destructive emotions of humans. The only difference is that spirits embody inward emotions, while demons embody outward emotions. Nor do spirits seem particularly intelligent enough to actually set up a longterm plan, they are just too single-minded.
It still seems most likely to me that the Elder One is one of the Forgotten Ones from Elven mythology, with Flemeth being one of the Creators. What exactly they are is still up in the air, because they certainly are not real gods, but they are powerful.