My headcannon for the Qunari is that during the War of the Gods one of them (the gods, I usually think Andruil) got the bright idea to mix a bunch of dragon blood into their followers. It made them raging savage lunatics, but good fighters (war fodder). They were feral, and after the gods disappeared there was no one to control them. After a while Koslun (or whatever his name was) came along, developed the Qun, and for centuries the Tamassran have been breeding the Qunari to be bigger, tougher, and more obedient. Selective breeding. That's my take on it anyway.
I used to have a theory that because of the spirit/physical duality of the ancient elves, they would adapt/alter themselves unknowingly to whatever beings they encountered, similar to the way a spirit takes on the traits they observe in people. And this was the reason elves coming in contact with humans would lose their immortality (becoming more human) and perhaps the Qunari were elves that lived so closely with dragons they took on dragon-like traits. I haven't discarded all of that theory in regards to elven duality, but since DA:I has suggested that Qunari might be more of an intentional breeding project gone awry, I've been thinking that theories like yours are more likely where the Qunari are concerned.
And while I may have confused "suledin" and "sulevin", the Qunari-ness of the "sulevin" concept still nags at me. The elven crosshatching looking like the Qunari symbol. The horned people pictured on the Par Vollen pyramids which happen to be built to mirror a constellation associated with Elgar'nan. Some of the concept arts for the ancient elves at Mythal's temple look like elves with horns. Were the ancient elves more diverse in appearance than modern elves? Perhaps not the same as Qunari, which would seem to have been altered, likely with dragon blood, but there may have been some other basis for the horns, since dragon blooded people like the Calenhad line don't have horns.
It's probably rare to unheard of, but I wonder what would happen if an elf reproduced with a Qunari. The Qunari themselves *are* carefully bred, so if something strange like the child of a Qunari and a human popped out completely human, it wouldn't happen often enough for a pattern to be noticeable. If an elf and Qunari actually resulted in an elf, that would be a huge red flag. Unless whatever's been done to the Qunari would override it. Hornless Qunari are rare, but when they happen, they're considered to be innately important or somehow more powerful than other Qunari. Could hornless Qunari be a genetic throwback to when they didn't have horns, and they still associate them with the fear and reverence they had for their original masters, even if they don't know that specifically?
Yeah, the idea that the Qunari were essentially some kind of super soldier breeding program of the ancient elves feels more likely to me these days. After the empire fell, maybe they were left stranded in whatever far off post they were originally stationed. Then after Koslun changed their culture, they sailed back home...