It is a known bug that Derkeethus, if married or recruited into the Blades, will not hang around your house/Sky Haven Temple. He will instead return to Darkwater Crossing all by his lonesome self.

Has anyone discovered a good reason why? It appears that he's dead set on following his basic packages- those that are assigned directly to him, not by Alias- but I can't place the reason as that alone, because it directly contradicts all other marriageable people and recruit-able Blades.

I'm examining it, and I can't find a reason. I figured I'd bring this issue to the attention of other people so the search can be as deep as possible.

Enter "StopQuest DarkwaterCrossingDerkeethusRescueQuest" in the console. That will cancel the quest aliases and remove his essential status.

The rescue quest grants him a rather redundant "go home" package that activates when he's not in Darkwater Crossing. The quest should be clearing if he finishes or changes from this package, but the scripting doesn't work.