Regarding Mythal giving Solas physical form:
If this is true, not only did she apparently bind him (he burned away the marks...) but it also means Solas never had a childhood, nor a parental figure or figures to guide him, other than apparently Mythal--who bound him. It's no wonder he's selfish. Over thousands of years, he has matured and grown and learned--but yet, in some ways he's still not any more mature than Cole is.
Regarding Solas's plans:
Solas does not appear to believe that shemlen (of any race) will survive, or at least that very few of them will. This implies that he either knows or believes that whatever was "broken" by the raising of the Veil can't be fixed for modern elves--but can be fixed for surviving ancient elves. Of course, we as the audience don't know one way or another. If the modern elves are reconnected to the Fade, and if some of them survive this, are they then Solas's people or are they still outsiders? I'm thinking they are still outsiders.
We also do not know whether--no matter how merciful he himself might wish to be to survivors--Solas has informed the other ancient elves that the shemlen are, in fact people. Or if he has, whether they've taken him seriously. We don't know what plans he will have in place for any survivors, or whether he has commanded/advised mercy to be shown to any such individuals. They may be the slaves of his reborn society, should any survive. Or they may have no place at all, and be pushed out wherever they go--or worse, killed on sight.
Regarding visiting Lavellan's dreams:
He's there to stare wistfully at his lover, that much is obvious. But he might have ulterior motives as well. If he's expecting the end of the world, he may want to save her one more time, even if she despises him for sparing her and no one else. He may also wish to grant her the mercy of a quick death before she can encounter a worse end in the chaos that results from his plan.
If he wanted to visit her to speak with her, he'd stay long enough for her to do so.
Regarding the Inqusiition:
I don't think Solas wants or expects you to disband. He tells you it's YOUR Inquisition. Maybe he's hoping the Inquisition will continue to exist so that it will be able to manage some of the chaos to come. Maybe he believes that in so doing, the Inquisitor might yet survive. Even if you hate him, he still respects you at least somewhat. But there's also the fact that, yes, he will have spies in your Inquisition, and will try to have spies in whatever comes thereafter. (Anyone get a Papers, Please vibe for the next game?)
He's expecting you to oppose him. Maybe he even wants you to oppose him. So why is it that he seems to want the Inquisition to continue? Does it figure into his plans somehow or does he, as I mentioned before, imagine that with the Inquisition, the Inquisitor might somehow survive? Does he not take the Inquisitor seriously as a potential threat to his plans?
Regarding Flemeth/Morrigan/Mythal:
It seems that we're setting out to oppose Solas's goals, however we do so--by convincing him, trapping him, killing him, etc. Opposing a villain in a video game is pointless if we can't win. So we're going to win. This is pretty much a foregone conclusion.
Here's the thing, though: Various previous in-game prophesies have told us that the Veil's going away regardless what we do or don't do. So if we do stop Solas, then... how? Are we too late? Or... maybe, does someone else do it?
Mythal has shown no objection to Solas's goals. She seems to have wanted to help him, even. It's easy to see Flemeth/Mythal as a force for good, nudging history down a safer path and rescuing our heroes from time to time at need. But her grimoire contained things other than just the body-snatching that Morrigan perceived as evil--and Morrigan was not such a compassionate person herself at the time. Flemeth/Mythal is just as morally grey as anyone else, and she's made no secret of her goals these past three games. Every time, she mentions vengeance. It's a vengeance she cannot have yet. She speaks of being betrayed herself (both as Flemeth and Mythal), and she speaks of the world being betrayed (presumably by Solas, but maybe it's more than that). We saw how well that worked out for Anders and Justice, right...?
So I am thinking that, if Morrigan is now to carry the essence of Mythal, she will be carrying out the plan even if we stop Solas. I don't know, maybe we'll get the chance in DA4 to even JOIN Solas toward the end?
My guess is that, in her quest for vengeance (possibly against Solas as well for betraying the world), Mythal will direct Morrigan to release the elven gods when the Veil is lifted. Whatever Solas did to them, he thinks was a terrible and fitting punishment. Mythal wanted them to suffer worse, and in her vengeance she is blinded to the harm she may cause--or maybe she just doesn't care about anything else, as long as vengeance is at hand. Remember, she began as a spirit, and although she seems to have been more complex than representing just one ideal, her primary association was with justice. Justice corrupts into vengeance, and Vengeance will have its due regardless of the cost.
What this might mean for the Sentinel Elves:
If Abelas is able to survive, (since you can have Morrigan stab him he would have to somehow survive this) then, having lived for so long with his world ended, he may not wish to join Solas. He might see that even if the shemlen aren't his people, they don't deserve the fate Solas has in mind. That's an optimistic view, perhaps, for us, and a pessimistic view for him (because it entails accepting that his people are already lost). In this case, he may join the fight against Solas or even serve as a double agent. His Sentinels would join you too.
If that happens, don't trust him. Abelas still has Mythal's vallaslin, which means Mythal is still presumably in control of him. While it remains unconfirmed that vallaslin formed a geas to the enavuris, Solas is said to have broken the chains of the elven slaves. Chains are not just symbols; they are physical bindings. Therefore it can be presumed that the vallaslin served as the elven slaves' chains, and they were freed by the removal of the vallaslin. If true, then whatever remains of Mythal can still command her sentinels. If Abelas survives and is commanded to betray you, he will. We already know Mythal is 100% on board with Solas's plan--she may be even more so, seeing as it's the only way to free the evenuris and get vengeance on them. Freeing them may also be a way to get vengeance on Solas for what he did to the world (unconfirmed that she wants vengeance on him for this, or even that the Veil is what she meant when she remarked on the world being betrayed). As I said above, even if we talk Solas out of it, the plan is going to succeed. The only other people in on it are Mythal and Solas's agents. Solas's agents presumably do not have the power, unless unified, to achieve his goals. Mythal does--and Morrigan will, if she has acquired a piece of Mythal.