Just going on what you've said above - if the qunari are connected to the blight - somehow corrupted or changed from the kossith by it - I think there's a large chance Solas could be involved there somewhere.
Like, their reappearance in Thedas could have something to do with that blood "decay" - perhaps it is slowly killing the qunari, or hurting them in some form? If so - it's possible that a more permanent solution is being sought in Thedas: maybe if they change/end the blight, they can change/end what's happening to them. Although I question how much world conquest has to do with that solution. Their arrival could simply just be a matter of imperialist expansion - grab more land, get more gold, gain new followers.
Although, that said, the dragon's blood theory points at what could be the root of the qun. Perhaps the kossith were dragon cultists, who began practising blood magic with dragon's blood - which had some very chaotic, horrible results which changed the race for the good - and the qun's approach to magic and order stems from that conflict. The qun does point at the idea that we're inherently corrupted, savage, and aggressive - Iron Bull states that his fear, leaving the qun, is that he'll become like that - and I wonder how much of that has a basis in reality. Was the transition from kossith to qunari so bloody that people genuinely believed that they were so innately awful? Something with a high risk potential like DRAGON BLOOD MAGIC could possibly lead to a situation where people thought as much.
There is something somewhere about Elves being susceptible to being converted to the Qun...Maybe Solas knows more about the Qun then others? He's pretty vocal about his dislike (to put it mildly) of the Qun and Qunari.
I think that's more to do with the social conditions most elves in Thedas inhabit - y'know, restricted to impoverished slums with very few prospects to improve their lot and the very real threat of racial violence if you try, if not enslaved to a human master - and much less to do with being elven. This is true even in the south (the slavery, of course, is illegal, but still occurs). The Dalish, of course, are a minority even there, who barely stray north of Orlais. For northern elves, the "traditional" elven ways aren't preserved in any form as an alternative lifestyle - there's no 'run away to join the Dalish' in the north.
Enter the qun, which provides purpose, promises stable economic conditions, and doesn't discriminate between followers. I think for someone without a place in society, and no other hope, that's quite promising. It just turns out, a lot of those people are elves.