The Globe review agrees with you - he liked the Reeves plot a great deal and felt the detective plot just got in the way.

By the by, the reason there is a resolution to the Elizabeth Short murder in the upcoming Black Dahlia film is that it is based on the James Ellroy book and not the actual events, as I understand it. The Ellroy book begins with the facts but goes increasingly into its own world as it progresses. It's a very good book, though, and the resolution he invents is at least a plausible one.