Yeah....I don't see how that's going to work out very well. You're already running all the NPCs, you know the entire plot - particularly in a mystery game like this, having someone on both sides of the screen is a recipe for disaster.
I just have an extreme aversion to DMPCs who travel with the party and act like a PC in all respects - I have never, ever seen it end well in the entire time I've been playing system games, and heard a vast number of stories where it ended extremely poorly. You're far, far better off just settling for DMing this time and PCing another time, rather than risking your game by trying to do both. You may think you can pull it off, but the odds are against you and it can torpedo a game faster than anything short of a DM's significant other.