I mean, it makes sense that he's investigating anything and everything since he's protecting his position. Obviously he can't just depend on trusting Ran, no matter how well he thinks of her, since that would potentially be putting himself in a fool's position. Covering all of your bases is an excellent quality to have for a leadership position.
Now, I understand why it might be unsettling for him to say he trusts her and do the opposite, but things don't quite always work out fine just because you think it will ^_^;
Ran bringing in Lumiere will probably instigate some jealousy from Yus though :3 It's safe to say that Yus won't kill her, but he'll probably 5ever keep tabs on her...