It depends on how you interpret what you're shown. Nocturnal never says that the Key is necessary for the Ebonmere to work, only that it's going to stay closed until the Key is returned. It's far more likely she's keeping it shut until her property is retuned to her. The Nightingales, as a Daedric Cult, aren't necessarily that reliable a source as to the motivations of the Daedra themselves. Another problem when looking at Tiber Septim is... He's a Reigning King (CHIM), and has almost definitely changed elements of his own history. So it's almost impossible to separate the original history from his revised one, and then you layer propaganda on top of that. However, Wulfhearth's and Arctus' involvement is integral to the enatiomorphic elements of Talos, i at least am more than willing to accept that Wulfhearth was Tiber's voice. We also know that some of the Nordic myths surrounding Tiber Septim are just flat out wrong. No dragons around? Hah! He had one WORKING for him. We also know Paarthurnaax was around. How did the Blades not know about Nafaalilargus though?