I'm convinced it's Gunter. Idk how the "warm hair, formal clothes, glasses" thing fits in, but I think that's all to distract you. What's important is the key, and Gunther is known for having keys to everywhere, so no one would know/find it suspicious if he had Mary's key. Plus, Twyla suspected Reggie despite the fact that he wasn't there at the time and she knew that, but she's too smart to not consider that so there must have been some way for him to have gotten there in time. And who was with him? Gunter. Anyway that's just my theory.
(Plus anyone with the combo of Sans eyes and cat ears is evil, obviously.)