Wotcha and welcome.
Dump valve - easily reverted to a recirc from a vent to atmosphere, yes they can make the car run rich as the air that has passed the maf means the ECU is pumping in fuel expecting it to all be there but it has just been vented where as the fuel is still being pumped in. MPG can also be dependent on your driving style. A bespoke map even with a standard setup can get extra midrange which helps overall mpg although I must admit I would expect more like 28 - 30 for your motor
Fuel filter - no idea for your model but they do have an intank one
green stuff on manifold - this could be the stuff subaru sprays all over the bay bay as a corrosion resistor but check your hoses and cap for any coolant leakage.
Rusty brake booster - Subaru metal protection is not great, I am powder coating a lot of my painted stuff but also use ACF50