I'm sure someone will be along with a better explanation but, for the moment:
On poppet valve engines it is normal to increase RPM then boost, throttle or MP.
I believe that his is to prevent overboosting and overheating of the exhaust valves.
When reducing RPM then reduce MP first.
An exception, in my experience, is the Bristol Hercules sleeve valve radial engines where we did the opposite and increased boost then RPM.
As you increase MP, the prop pitch will coarsen in order to absorb the greater power and produce more thrust.
Re optimal settings: there should be guidance in the flight manual.
The plus point, when you make it onto the Dreamliner, is that you don't have to worry about all that stuff
