Doesn’t sound like realistic behaviour to me. What you are talking about (too high RPM) can’t be solved by backing off on the throttle, eventually it will when you reach the point where the engine isn’t producing enough power to drive the propeller and RPM will drop.
You’ll need to move the propeller speed lever back a bit, this lever controls propeller (and engine) RPM by changing the propeller blade angle. With it set full forward the propeller is in a fine blade angle which results in high RPM. Moving the propeller lever back commands the governor to maintain a lower propeller RPM by changing the blade angle to a coarse pitch.
The manifold pressure issue you are describing sounds like a bug, unless power is set really low (idle or close to) the MAP should not be close to zero. I don’t know what the auto-MAP in MSFS does but I’m assuming it keeps the MAP constant during climb and descent. Normally you’ll need to add power during climb and reduce during descent to maintain a constant MAP.