I accidentally posted a post too early and when I finished writing it the thread advanced two pages >_>
I'm still 99% convinced that the Veil will go down no matter what, whether by Solas, or by some other person.
In my eyes Solas doesn't need to do anything to big to redeem himself (like throw himself to the pit of whatever so a green light would wash over everybody and make them spirit hybrids capable of understanding spirits and demons), putting on the Veil was a mistake that cost a lot of peoples lives and the lose of a culture, but we can't really know how dire was the situation, perhaps the Evanuris were a step away from destroying the world.
I imagine it could go several ways:
Irredeemable Solas:
1. Low approval Solas, Inquisitor pledged to track him and kill him.
2. Solas removes the Veil without hesitation.
3. Perhaps chaos indeed ensues, most live through it, Thedosians are, as Felassan says, stronger than they look.
4. Devastated Solas realizes he cannot rebuild the elves, since they are vastly outnumbered and now they made enemies with what is left of Thedas.
5. Solas is killed for his crimes.
Redeemable Solas:
1. High approval Solas, Inquisitor pledged either.
2. Solas' lack of conviction makes him slip, allowing Inquisitor & Co. to realize the repercussions of removing the Veil.
2.1. We find a way to remove the Veil safely OR we find a way to prepare for the removal of the Veil.
3. Solas is perhaps approached with said ideas, we [don't] manage to get through and cooperate with him.
4. Veil is removed safely OR we set a goal to operate an organization that will help deal with the issues caused from the removal of the Veil OR Solas decides in some miraculous manner that the world is worth keeping as it is, Mythal or perhaps an Evanuri that breached their "cell" removes the Veil.
5. Solas is judged as players please. A redeemed Solas helps the effort of stabilizing Thedas after the removal of the Veil.
I personally tend to believe Solas won't back out too easily from removing the Veil, I don't think he is capable of seeing the current world as beautiful as what was (and I kinda understand him). If I had to guess how Inquisitor/new PC could kill Solas I would bet on a side-effect from the sundering of the Veil.
Also, a mix could work, Inquisitor & Co learn of the repercussions and prepare themselves regardless of Solas.
Of course this all could be nonsense, I can't know the plot for the next game, nevertheless, I am still (almost) certain the Veil will come down, too many prophecies foreshadow that (Sandal, Flemeth, Eleni, Canticle of Exaltions [Prophecy]), it would be silly if nothing would change...
Plus, it's a chance to create the most beautiful animated CG in gaming history XD