Units that are summoned in battle disappear after the battle. So, even if your mind control stuff had worked, you would not have kept them. (iirc)
Horrors do have high MR (20). Without penetration boosters you are not getting them that easily.
http://dom3.servegame.com/wiki/Horror
Also, horrors have the (attackfriends) tag. I don't know what that does. I think it makes them attack after a certain while. So horror mind control isn't that great of an idea.