I think it's more interesting to debate Orlais' fate because everyone agrees that the City Elves deserve more than they have, but is it worth destroying a nation to get rid of its flaws?
Sometimes. But the assumption there is that destroying the nation will get rid of the flaws. There are few examples of that being successful, and those that are are when the flaws are the result of an empowered minority. Aspects with widespread cultural acceptance will exist without the state, and even survive against state attempts to remove them. It takes extreme patience over time to convince societies to change their common views peacefully (see the decline of influence of Catholicism in France), or extreme brutality and eradication to remove dissent in the short term. Neither are guaranteed successes either.
The question here is 'what do elves deserve'? Defining that will affect the appropriate courses of action.
If the elves 'deserve' their own lands away from Orlais, then yes- taking advantage of Orlesian weakness is an appropriate course of action. Especially if Briala uses the Eluvians to evacuate elves from alienages to some new elven homeland. It is risky, and in no way guaranteed to succeed, but it has less chances of failing while Orlais is temporarily weakened. Societal segregation makes sense with a strategy of prolonging and perpetuating the civil war.
If the elves 'deserve' equality and integration into Orlais and broad respect from humans, however, then perpetuating the civil war and making an enemy of both factions along racial lines is a Bad Idea. No matter who loses, the winner is still 'the humans' who will turn their attention towards reasserting influence over the elves in Orlais. The elves who have spent the time fighting humans and not helping. The elves would be far better making a bargain and embracing one side or the other as a relatively autonomous ally- there is the risk of choosing the wrong side if they jump in too early, but the later they hold off the greater their chance of missing on the chance to be an integrated and indebted ally and more about being an eleventh hour opportunist. The later is better than being an enemy, to be sure, but it does less for integration and respect.
Destroying a nation to get rid of its flaws is just part of the nation building conceit.
As for the Dalish, I take issue with the people who are apparently unable to accept that they have big flaws as well, as if Zathrian never existed. The fact that they lost a war and are victim of an ally's betrayal way back in history shouldn't make people blind to their flaws. Apparently, the elvhenan empire was, if at all, only marginally better than the Orlesian one, and some of the Dalish are perfect reflections of that fact. This really shouldn't surprise anyone.
I wouldn't even call the Dalish conflict a betrayal. Not until we get something that can credibly justify the sacking of major Orlesian settlements.
(In a more general sense, it does not matter for the moral evaluation of a political evil whether or not a class of oppressed people is defined by race or some less visible attribute)
I'd disagree. The moral evoluation of political evil should factor in the nature of differences, visible or not. One of the reasons racism is stupid-evil is because it rested so much on pseudo-science to invent differences that weren't there (or at least mis-attributed).
If a class distinction is made on the basis of a generally non-visible but real attribute, the nature of that attribute will effect the morality of it. Being an violent insurgent, for example, is generally not a visible attribute, but it is significantly different from a peaceful dissident. Even if you can't tell the difference between the two visibly.