Make sure you save before trying THAT particular experiment, as summoning the demon destroys the armor. If it doesn't work, you've lost yourself a darn good piece of armor for nothing. Even if you don't want to use it, it's still worth tens of thousands of gold at the store/temple.
Did you try the export/import method? Yes, some would consider it borderline cheating, but I really don't think there's a way to kill off a protector of the woodland and loot his twitching body without it being horrendously evil.
If your concern over gameplay integrity trumps your greed for the best longbow in the game (by far), well done. If not, so what?