it's pretty easy to forgive a detective who doesn't know what genre he's investigating to start with non-supernatural possibilities before moving on to "the victim was manipulated into resurrecting his alchemist ancestor who looks just like him, and who then killed and replaced him."

I can see even detectives who are open to the supernatural not getting that one from the start, honestly. It's pretty specific.

I suppose as a movie, it's not really that noir -- Curwen's quite evil, but no one else is really that bad.

I don't consider wall-to-wall terrible people a prerequisite for noir!