You pose several scenarios.
Is more cargo added if people do not show up?
No. By the time a person does not show up, the cargo is usually already delivered and loaded on the aircraft.
If the plane is over the estimated weight - does cargo get removed? Passenger bags removed? Passengers removed?
Yes, although passengers almost always have priority. On heavy cargo flights, the cargo department will send a priority list if the load planners need to bump cargo.
Is it within the airline's strategy to achieve 100% seat capacity utilisation?
Absolutely, although on payload restricted flights, non-revenue passengers may not always priority over cargo. This depends on the route. Many long-haul flights, if payload restricted, a scenario could happen where a staff member traveling as a non-revenue passenger will not be accepted because accepting them would exceed the max payload for the flight.