Unless you have a turbonormalized or turbocharged aircraft, your manifold pressure can never be higher than ambient pressure, only lower. You regulate this by closing and opening the throttle valve.
This is close. Manifold pressure in flight with an open throttle plate will be higher than ambient pressure, assuming an unobstructed induction, because of ram air increase and a slight pressure increase from the propeller.
Rolls Royce invented a two speed supercharger for the Merlin X engine. The low gear speed was used at low levels and the higher speed for higher altitudes.
Two speed superchargers are standard on nearly all supercharged aircraft engines...though many operators today restrict operations to the "low blower" position.
If one takes the power v. fuel flow graph for a given engine e.g. the IO-540 and extrapolates one of the constant-RPM lines to the zero-HP intercept, one finds several GPH fuel flow at that point. This fuel flow presumably represents the total of the pumping and friction losses, at that RPM. It is pretty significant, and explains why flying at a lower RPM, say 2200 v. 2400, does produce a significant improvement in MPG.
Significant differences exist between fuel controlled engines and carbureted engines or those utilizing simple injection. Fuel efficiency and engine efficiency at lower RPM's is largely due to the effects of a constant speed propeller, rather than simply slowing down the engine...but this is largely irrelevant when considering manifold pressure on it's own.