You will get the greasy build up on the throttle body because of the PCV system. Which hose is split? The PCV valve is behind the intake manifold which is why it costs so much to replace it. The PCV valve is very simple with not a lot to go wrong. Mine even held up to boost before I removed the system completely.
The hose that goes to the valve cover carries already metered air into the top of the motor and the air rejoins in the intake manifold so if your venting already metered air that could be playing into your bad mileage.
I would recommend the plugs (which you've done) and a fuel filter change. I have never got less than ~25 mpg since I started driving my first '05 Focus back in 2007. My ST even got over 30 mpg highway with the turbo on it.
'07 ST ; formally bewst'd
'86 XR4Ti ; 2.3L Turbo, 15psi
D23 Crew: #07