I'm not going to overly debate the terrorism vs justified acts of revolution bit too much concerning Anders, but I think you're speaking way too generally here. Anders's actions may have made it impossible for some people to stay with him due to their own beliefs and values, but certainly not everyone. My Hawke was happy to stay with him, and he was happy to stay with my Hawke. They shared a touching moment at end game in which they affirmed their love for each other and agreed to run away together. I assume we left with my BFF Isabela on the ship I helped her acquire, as I spoke to her about. In DA:I, it was made clear they're still happy together and had been working to help liberate more Circles. This shows they not only worked things out, but now Anders trusts Hawke to help him with his work without having to hide anything. I'm sure my (still alive) Hawke will return to Anders soon enough. Anders's actions didn't make it impossible to stay with him, it's just a matter of perspective. Hawke and Anders can have one of the more clearly defined happy endings for DA couples, if you pursue that. Solas's actions legitimately made it impossible.. because he left and refuses to let you be a part of his plans. Even if you wanted to stay with him, he's made it impossible.
I guess my point was that marriage shouldn't be the end of every relationship: sometimes, things go wrong, or staying together is impossible. If you think that every romance option should end happily ever after, then you are arguing that Dragon Age should be more wish fulfilment - and Inquisition was the most wish-fulfilment of them all. I don't think Dragon Age should always end happily ever after, I think there should be tragedies, there should be bad endings, there should be romances that don't work out, and in that regard Solas' arc is very well done. He has a mysterious disappearance like Morrigan, but he also has the villainous deeds of Anders.
I didn't romance Anders - but I thought he was the worst character of the worst roster of companions in Dragon Age. But, I guess, that's neither here nor there. I still have absolutely no idea how anyone can justify his actions in blowing up the chantry, but again there are countless other places to discuss that. Either way, I am sure that, for a lot of people who romanced Anders, those actions really did put an end to things, and that would feel frustrating - it felt frustrating to me and I didn't even romance him. You can't really assume that most people would be OK with what Anders did, given the chaos that ensued and the countless innocents who died. Just because your Hawke was able to ignore that, you think that it is a simple choice for the player to make, and I guess it is if you aren't really much of a roleplayer.
The difference is that Inquisition puts Solas in the role of antagonist before he puts his crazy plan into action - it doesn't have him murder everyone then ask you if you're ok with it. The relationship is over, and there's no going back.
I think you are really jumping the gun on assumptions of Solas and Mythal's relationship, though. Maybe they were lovers? They never said that in the game, though. I don't think we know what's going on with them. It seems very quick, to me, to suggest that Solas could never love the Inquisitor because he still loves Mythal. We know he was moved to create the veil because of her death, but why beyond that is assumption. Maybe they were "together" in that way, or maybe he was just loyal as a friend. Maybe he just hated their betrayal and saw there wasn't any chance of correcting everything wrong with their society without her leadership, or that they would just keep killing anyone that spoke out against them if they would kill her. To me, the "Solas made the veil and can never love again because Mythal is his true love", story take some filling in of gaps that still exist with one's own assumptions. That would be a very cliche outcome, so it wouldn't be totally shocking, but a little disappointing, imo.
I know that I'm jumping the gun but there isn't a lot that's been revealed to us, so speculation is all we have right now.
I think it's fair to say Solas loved Mythal. He essentially destroyed the world after she died - there is no way that wasn't an emotional decision, especially given how emotional he is when he reunites with her (Flemeth). The only question is: what kind of love?
I think their relationship was one of unrequited love - we know Mythal was the wife of Elgar'nan and bore his children. We also know that Solas was regarded as both an Evanuris and a Forgotten one, able to move between groups, something which could have made it so that, even if Mythal reciprocated his feelings, it was impossible for them to be together. Also, his fear is 'dying alone' - yet he forces himself to be alone. I think this is because he is still grieving for his true love, Mythal. Everything about Solas, from his cadence, to the gradual revelation of his backstory, to his attire, which in inquisition is that of a wanderer, and that of a warrior in Trespasser, to his fantastic Trespasser theme, tells us that he is a sorrowful character, one who tried time and time again to do the right thing and has faced the ultimate punishment - the death of his loved one, and the death of his beloved nation.
I remember Solas' line, that he'd awoken to 'a world gone tranquil'. I think, when it comes to Solas, quite simply nothing of modern Thedas can compare to the Elves in their heyday. Some things may come close, but even Inquisitor Lavellan cannot match Mythal. That is why he is working so hard to undo his mistake - he is not, in his mind, destroying the world, but undoing its destruction at his hands. That is also why he refuses the Inquisitor if she offers to accompany him, a decision which would make no sense if he truly loved the Inquisitor, and no-one else.