Good points. I was more leaning towards Tevinter watching spirit abominations closely as research. Because obviously while still dangerous, a spirit abomination is a lot safer than a demonic abomination to have around and "study".
Maybe with a lot of effort you could create a possessed mage who is absolutely in control, a winter soldier, or in this case a winter mage. Of course it would take a lot effort and at best 1/10 of possessed mages could reach this state, but would it be worth it?
Something akin to Seekers, except they would be a mage.
I doubt control is something you can breed like you are seeming to suggest. And even then, it's never a guarantee that you wouldn't corrupt a spirit down the road.
Also, Seekers aren't possessed indefinitely, not like you seem to want the mages to be. The Spirit touches them, breaks the Tranquility, grants their powers, and then it leaves.
I imagine some in Tevinter have tried for the perfect being, and as others have pointed out, it ended terribly for those involved.