I'm also of the opinion that the Veil is probably an Elven construct, I also believe Arlathan is possibly the Black/Eternal City. I also am thinking the Blight was contained by someone in the Eternal City. I'm also of mind to believe Solas involved. Yet, there are a few things that don't sit right that maybe one of you could assuage.
First is the god Falon'Din and the concept of Uthenera. Largely, these two things are rooted in the division between the Fade and Thedas, and the mysteriousness of the Fade as the place of death for an "immortal" race. Falon'Din was said to be the only God who could go there, and his drive to amass followers led him to kill many, many elves (presumably because as the God of the Dead, the souls of the dead were within his sphere of influence). Maybe we are missing important details, but if Solas created the Veil, dividing Thedas from the Fade/Beyond, why would Falon'Din be represented as he was (even in DAI) and why/how could Uthenera be a thing where Elves left their physical bodies to bum around the Fade for a while?
Next is Mythal. Flemythal says that she was betrayed as the world was betrayed. Abelas says our favorite goddess in the body of an old hag was killed. As the Fade and death are linked, and as the story of Flemeth involves an entity from the Fade, and Flemeth was probably gifted with magic before joining with Mythal, what constitutes death for a God is a severance from the physical world and being unable to bridge the physical/non-corporeal schism by normal means. To me that means that the betrayal Mythal experienced, that was akin to a betrayal of the world, had to do with a fall, a death, a sundering. Sounds a bit like the Veil being thrown up to separate a recently killed Mythal from the physical plane, and as we know the Dread Wolf was not responsible, this makes me think Solas was not involved.
Thirdly, Solas and Mythal's daughter Yavana seem anti-Veil. Solas speaks of the Veil as the apparatus by which Fade entities and mortals are doomed to misunderstand and destroy one another. Yavana speaks longingly of the time before the Veil when dragons ruled the skies. As Solas and Mythal do not seem to be enemies, but rather working alongside one another (until one of them decides their goals are more important), I don't know if I can get behind Solas simply regrets the creation of the Veil.
Next is Solas and Corypheus. Presumably, if Solas created the Veil as an apparatus to imprison (possibly tainted) elven gods, why would he "give" his orb with the potential to breach the Eternal City to one of the Magisters? Solas knows a lot for having slept for thousand(s) of years. Presumably he would be aware of who tried to break into his little prison before, and would not give a powerful object to such an entity unless he was unaware of the properties of the Blight (then how would the Eternal City be a place to lock away the Blight as designed by Solas?), Corypheus hid his tainted nature (seems like it would be hard to trick an immortal entity who is possibly paranoid of the dangers of the Blight), or Solas knew Corypheus was tainted, and thus had tried to breach the Eternal City, and wanted him to try again. Sounds a little dangerous to let a prideful Magister have your key to the prison you designed that said Magister tried to break into once before, unless you're extremely confident, or you want them to do just that.
On a related note is Solas and the Wardens. Solas seems to not like the Wardens much. He especially does not like the Wardens plans to kill the Old Gods. He believes the Wardens to be toying with things they barely understand. No where does he seem to be affraid of the Blight. In fact, he seems unfazed by Red Lyrium throughout the entire game, and if the Blight had really been why he imprisoned his peers in the Eternal City, would he not be mortified of the Blights prevalence in Thedas? Solas even explicitly states there is no "known" link between the Old Gods and the Pantheon. If the Blight was somehow plaguing the Pantheon, and had corrupted Old Gods causing Blights since the fall of Arlathan, would that not be a link? Another thing, Nightmare, an entity that feeds off the fear of the Blight knows Solas. Why? I also feel like it's too much of a coincidence that an entity that feeds on the fear of the Blight would use the word "banal" which is a root of the word for the Blight in Elvish (banalhan) and Solas would also use it. Maybe that is a stretch.
Finally is the Eluvian from the Dalish Origin in DAO. As of DAI we have seen waaaaay too many Eluvians. All of them have been dormant and needing of a key of some form to open. Except that first one. It was activated and spewing out the Taint. Now, there is no way to know, but as the Brecilian Forest is not readily known as a place to encounter Blighted creatures or the Darkspawn, it seems likely that Eluvian activated relatively recently. Also, what Tamlen sees in the mirror is probably the Black/Eternal City. If said place is a prison, and someone activated an Eluvian leading there... who? Why? I don't think it would be Flemythal as she seems pretty anti-Blight corruption and admits the Blight would consume her as well in DA:O.
Maybe I am over thinking things, but I feel like these things don't sit well with the whole Fen'Harel was the heroic bro man who sealed away corrupted gods from destroying the world by creating the Veil.