BMEP = Brake MEAN Effective Pressure - Is a theoretical pressure developed inside the combustion chamber that if maintained equally throughout the two or four stroke cycle will develop a set horsepower. It has ABSOLUTELY NOTHING to do with MANIFOLD PRESSURE!!
You're right.
Nope, RPM is not the only factor governing manifold pressure but it is the only factor that governs supercharger output and with a constant throttle position, which is supposedly what we're dealing with here, would be the only thing that would affect manifold pressure!
RPM is the primary factor determining supercharger output at a constant throttle setting when the engine is developing power. It is not longer the determining factor after a power loss.
A supercharged engine developing 49" of manifold pressure during takeoff, for example, will not maintain 49" of manifold pressure, or the same supercharger output, after a power loss, even though the same propeller RPM remains.