I certainly hope the main mystery is a fun element for the player. If it gets lost in all the porn, you're right, it dilutes the theme and story. That said, I intend to have lots of mini-mysteries/challenges with most of the characters, but with the intent that every one may provide some little nugget of information involving the main mystery. My goal is essentially a new vignette with every release (like the last one was with Rita, the one prior to that was Polly, and Zoe before her).
Anyway, the idea being, based on all the main clues one acquires, the player will detail why and how the crime occurred (there is more than meets the eye, from what he witnessed). It will be possible to be wrong, right or have elements of both, based on what the player picks up during the game. I hope to code a "clues" game where the player takes all the clues and ties them together, based on what they are linked and associated to. In the end, if the player gets everything exactly right, there's some big "win" reward. If not, it will be more like the ending of Chinatown, and life will still move on.