(As per usual, sorry for the rambly wall of text.)
- Bringing down the Veil - it might destroy him, as he hints in Trespasser, or it might not. He might succeed in bringing it down; he might not. If Inquisitor convinces him not to tear down the Veil and he can't restore something of the ancient Elvhen at all, can he be happy living in the modern world, Veil intact?
I dunno. Can he be happy living in any world? Well, I mean - hmm. I think Solas could, conceivably, live with Lavellan, living as he pleases, being happy and fulfilled. But I don't think Solas could ever be content. He doesn't seem the sort to just give up and stay home when he thinks something's wrong, and thinks he can help fix it. He doesn't seem the sort to give up and stay home, period. He says he could never simply stand by and watch the Breach consume everything, and while I'm sure that's at least partially from a place of guilt, I think it's also just his character - It's why I'm always sort of annoyed when I read a fanfic that gives him some generic happy ending, with a house and a lawn and 2.5 kids with Lavellan. It doesn't seem like the sort of life Solas would be content with. He likes to travel, to learn, to teach. He revels in court, he debates with the companions, he takes a stand for his beliefs and values even when it'd probably be best for him to keep quiet. He tells you that story, of him as a small child, who leaves his home because he's learned all he can, and seen everything there is to see.
To me, that doesn't sound like someone who'd be happy with the typical life, being a husband, raising kids, happily ever after. So... I dunno. I can't see it, this fairytale ending. I think he could achieve a happy life. But never a quiet, peaceful one - he's too restless.
- Rebuilding the ancient Elvhen world - he can only rebuild it based on his memories and perhaps the memories of other surviving ancient Elvhen - and it's well established by Solas himself that memories are flawed (hence why the past events he sees in the Fade can have conflicting details, like his description of the Battle of Ostagar. Yes, it's also a way for BioWare to accommodate different players' world states without busting the word budget, too, I know.
) Would that world be comfortable for him or would he find it deeply flawed/missing something essential?
I don't think he cares too much about replicating Arlathan. I think he's more concerned with the return of magic and spirits and immortality. Maybe he'll want to restore certain landmarks and wonders, but the civilization as a whole? I mean, he rebelled against it, after all.
- Being immortal with a potentially very mortal Lavellan - If he can't transform modern elves into immortal Elvhen, where does that leave his relationship with Lavellan? I think it would be hard for him to watch her grow old and die while he stays looking about 45 years old. If he and Lavellan are physically intimate, will he eventually find it somewhat lacking for him since the ancient Elvhen seem to have flipped back and forth between physical and spirit forms, flying around, etc. while getting intimate?
Immortality came from the Fade. It's currently blocked by the Veil. And given the hints about elves in the Crossroads that we got from Trespasser/Masked Empire, I think this remains the case even for modern elves. So I find it likely that, if Lavellan somehow survives and her connection to the Fade is restored, she'll regain immortality (And magic if she's not already a mage.) Even if she doesn't, I don't think this would prevent Solas from being with her. It'd give their romance a more tragic ending, sure, but her mortality didn't stop him from getting into the relationship in the first place.
As for changing forms, Lavellan's already capable of all that - in the Fade. We know she (And all other Inquisitors) are capable of rudimentary Dreamer abilities. We know anyone with proper control over themselves and their dream can change their form.
Like - you ever wonder why Solas seemed so happy and surprised during that Fade scene? Why he tells the Inquisitor "You continue to surprise me?" Why Weekes said that, if there's a point where Solas truly begins to see the Inquisitor as an equal, it's during that scene? Because the Inquisitor proved they're capable of all that, of everything Solas does, of everything the ancient elves used to do. They can walk the Fade, they can speak with spirits, they can visit memories, they can recover what's been lost - oh, sure, they're not very good at it, but that's just a matter of time and practice.
That was the whole point of the scene - there's the world Solas loves, this world he thinks has been slandered and broken and irretrievably lost - and here's Lavellan, who, with a single dream and a few words, proves him wrong. Proves that there's still hope, for the Fade, for mortals, for this world. So... *Shrug* I don't think that'd really be an issue.
- Guilt - if his plans succeed but do result in the deaths of many modern people, can he live with that guilt? If his plans fail, can he live with the guilt he's already feeling for having destroyed the world of the Elvhen?
Ah, now this, this is the only serious problem I see. Because, yes, Solas is going to be burdened with guilt no matter what he does. I imagine the redemption arc will deal a lot with untangling this guilt, but if (As the game implies) there are people still currently suffering from his actions, than I don't think a pep talk will really cut it. Convincing him to let go of the past is one thing. Telling him he needs to abandon living, breathing people who are trapped in torment because of his actions? That's going to be a harder sell.