Not that it destroys it. That it is lethal. It's different. And I don't believe it is Alleria who says it, it's only mentioned in her audio drama.
Xhul'horac can indeed be dominated. Gul'dan was controlling him to support his war effort. Xhul'horac was ultimately overwhelmed because he couldn't withstand the amount of power he was channeling. The entire theme of the Ren'dorei is learning how to control such overwhelming power. As their story progresses, so could their knowledge on this subject. Besides, Xhul'horac was being fed with overwhelming amount of energy by Gul'dan. If Gul'dan wasn't spoonfeeding him to no end, he probably wouldn't have been so unstable once unleashed.
Besides, I never said other races shouldn't be demon hunters. Only that Ren'dorei demon hunters would be an interesting combination and lorewise would advance the Ren'dorei's story in a unique way that at the same time respects established characteristics of both void and fel. Also, the Void can bend fel creatures. We saw that in Mac'aree. The broken there, who were previously mutated by fel magic (that's how draenei become broken), ultimately succumbed to the corruptive power of the void and were being controlled by the Dark Naaru.
Finally, demon hunters are not and never were a race.