1. There should be an expansion tank in the system to account for variations in water volume based on temp - the pressure should be stable. 20psi is on the high side of normal (often around 1 atmosphere - 14-15# - but usually at least 12...there's a pressure relief valve usually set at 30psi that will dump water if the pressure gets that high).
2. Every system is slightly different. You may need to run more water through it if you don't have heat to all zones to purge out the air. If there's heat everywhere, any small amount of air left should get purged if the air scoop or extraction device is working properly.
3. Dropping the pressure likely caused the boiler water to flash into steam - mini steam gas explosions. Long term, this will cause problems similar to what happens if the timing in your car is off or you don't have high enough octane and it pings or what would happen if you opened the radiator cap when the coolant was at 230-degrees. If everything works, you're probably okay.