On how much he looks down on others:
I think if he hates anything it's the culture. He clearly cares about people as people but he can't stand any of the cultures of the present. I do think he sees the current elves as "lesser" though. I said in another comment that saying he made a mistake, without acknowledging positive changes, is kind of saying that the slavery of his time was better than whatever they have now. He also seems perfectly willing to blame the Dalish for what they are. Never mind that they inherited their false beliefs, it's the current generations fault that they believe it.
I think Sifr's comment about him playing chess with outdated rules and half the pieces is pretty accurate. There's a lot of history he just doesn't seem to care about that affects how things are for the elves now.
On the metaphor:
The difference is that the Dalish mistakes aren't killing them. What's killing the Dalish is their lack of unity and probably no small bit their arrogance. Being arrogant about something false isn't really all that different from being arrogant about something true in this particular case though.
Also, where's his proof? He seems to expect people to listen to him just because. He can't actually prove anything he says about ancient elven culture. And why would it be better if they knew the truth? He cares because it's wrong but the Dalish have turned the falsehoods into a form of strength. It may not be true but the beliefs unify them as Dalish.
We'll probably have to agree to disagree on whether his frustration with the Dalish comes from their willful ignorance or just the ignorance itself. I still thinks he hopes that the Dalish will come around. In the same way that I hope my sexist relatives can one day stop being sexist and join me "in the light." Maybe that's what's going on -- I'm projecting a bit because I can sympathize with Solas as a character that sees something so clearly while everyone else stubbornly clings to something that seems so wrong to him.
Anyway, regarding whether or not the Dalish actions are harmful:
I think he has proof - somehow - but he won't show it because it would be bad for his plans. If he shows what kind of proof he has and the Dalish go "That's all fine and well, but even so, we don't want your way of living. We are fine on our own and will learn from the proof that you showed us.", then that is not what he wants. The Dalish and all Elves would, at best, learn from what he showed them. But they would certainly not go back to the time that Solas deems perfect. Why? Because the Dalish themselves do not belong in that time...we don't even have to talk about the City Elves. He himself doesn't even acknowledge the current elves as his people, so why would they even want to go back to that time when they were just slaves?
This is the part that makes me mad/sad. How do you explain something that you know someone can't comprehend? I keep thinking of the parent/child line of analogies. If you know that something is better for a child but they still resist do you make them do it anyway? I can't count the number of times my mom forced me to wear a coat going outside because she knew I'd be cold even if I didn't believe her. Sure this is much bigger than that, but I think Solas' line of reasoning is the same.
The Dalish believe that their Keepers are descendents of the elven nobility, so they might very well want to go back to Arlathan. They would probably think that they would be the ones in power, and there's no shortage of Dalish that believe they're better than the other elves. I would guess that unless Solas flat-out told them "you're going to be slaves," they would assume someone else would do it. The "not me" syndrome. It's easy to trade someone else's freedom when you think you're getting an all-powerful magical empire in return.
I do think he sees himself as a tragic lone-wolf character leading a rebellion against forces for the betterment of the people too ignorant to understand. I don't like it, and I wish he would "settle" for the more mundane route of slowly working with the Dalish/City Elves to build a new society. It would be harder, and probably take thousands of years, but it would show that he understands that one can't just bring back the past and hope everything will be OK. And that he can acknowledge the autonomy of the elves even if he doesn't believe they are taking the best route.
Re: a sex scene
Maybe I'm in the minority here - when I played through the Solas romance, I didn't actually register the lack of a sex scene. I didn't really notice that there wasn't one and it didn't bother me that there wasn't one. I can't really think of what a sex scene would add to the romance that isn't already there - the sense of intimacy between them, the depth of emotion, etc. I don't think a romance needs to have a sex scene in order to be mature, deep or fulfilling.
I do think that the writers went about sex in the Solas romance in the right way - leaving it up to the individual player to decide, more or less, was a good idea imo given Solas' true nature and all that that entails. There is certainly enough allusions to Solas and Lavellan being sexually intimate present anyway to support them having that kind of relationship if you prefer to think of them in that way.
I'm not sure how I feel about future Solavellan content (ex. DLC) having a sex scene included if I'm honest. All of the other female-available male LIs (Cullen, Iron Bull & Blackwall) come with a sex scene attached to the romance iirc? I'm not asexual but I think it is important for at least one of the female-available male LIs to not have a sex scene associated with it. I don't like the idea of all romance paths forcing sex between the PC and the character on the player. Iirc there's no sex scene with Josie and I like that there's a female-available female LI that allows for this.
I wouldn't mind a fade-to-black scene in future Solavellan content. No one has to be shirtless, they just start kissing and then it fades out suggestively. Part of this is because I don't think BW can animate things that will be as vivid as what fanfiction writers can put out smut-wise, and part of it is because like you said - Solavellan is more intellectual/spiritual than physical I do find it interesting that Josie and Solas - the two characters I'd assume as most interested in procreation (Josie because she's going to be the head of her family, Solas because he's trying to save his people) - are the two that don't have sex scenes.
I wish that in lieu of a sex scene we'd gotten something else though! Solas reading love poetry has been tossed around. The change in intimacy between scene 2 and scene 3 is huge but there's nothing in-between to indicate how that evolved. I wish they'd put a cutscene in there that showed them becoming either more physically or emotionally comfortable with one another.