I guess what irks me is this attitude that people would totally romance him if he LOOKED a certain way; never mind his personality, intelligence, and interesting backstory. 
I know that feeling. I get a bit annoyed whenever someone says they'd romance Solas if he just had hair but then I remember that I'm not really one to talk - like, I just can't get past Blackwall's beard. I can't. Maybe if his character resonated with me more, but not as he is. It's not that I dislike all facial hair - I can appreciate a trim goatee (Like, damn, Hanzo from Overwatch pulls it off pretty well, and I may or may not occasionally use Adam Jensen's face as my desktop background.) but that lumberjack beard is just... I can't. I want to. But I can't.
I also have a hard time really appreciating Solas art that gives him dreadlocks. I like the whole... side-cut ponytail hairstyle, but dreadlocks are just a huge turn off for me. So it'd be rather hypocritical of me to fault people who feel the exact opposite. 
I can't really speak to his logic because... well, it's his logic, but I think his reasoning was less "The Inquisitor's dealt with no hardship or true obstacles up to this point" and more "their victory would've been 'free,' since they lost nothing they could not eventually get back."
I think he was thinking along the lines of Earn Your Happy Ending (Which admittedly is a pretty poorly defined trope), that without Trespasser, victory would've come with little cost, since no real sacrifice is ever made or asked of the Inquisitor (Compare to the Warden, who has to make the choice with the archdemon, and Hawke who... well, Hawke.) Like, the only real sacrifice you make is in the Fade, and really, that's a sacrifice being asked of the player - the Inquisitor has no real stake in these people, not to the degree that the player does. Even Haven, devastating as it was, only targeted nameless NPCs, minor side-characters, and that one Chantry guy who's job for most of the story was to just be an jerk. To him, that sort of... cheapened the victory, I guess? I dunno, I think that was his reasoning, but I could have misinterpreted. *Shrug* Still, I agree that it's particularly bad for Lavellans though, even if you don't romance Solas. Temple of Mythal definitely would've been a punch to the gut.
Plus, I'd like Solas to get something right for a change
I joke about him being the God of Bad Ideas, but you know, variety's the spice of life.
I'm pretty sure Mythal's aware of and encouraging Solas' plan, but I don't think she's fabricating information and directly manipulating him into doing it. Just... being opportunistic. She may be using him, but I think he decided on this course of action all on his own. (After all, he didn't even know she was alive until the Temple, but he's been on this path since he woke up.) Not to mention... can't Flemythal make that whole "Reckoning that shakes the heavens" motive rant while Solas is in your party? Standing right behind you? Probably paying attention to the elven goddess making vague but ominous declarations of change? So... he's probably aware that she's got plans of her own, and assuming he's not in on them, he's probably at least looking out for it.
I do agree that they're going to have to give him some major victory or two if they want the player to take him seriously (Both as a genuine threat and as a competent character.) But, seeing as how they acknowledged that as an issue with Corypheus, they'll probably be sure to not let it happen with Solas.