A pump is used to move water. Water is not compressible in this system. If you have an open system, like from one bucket to another, the pump can't achieve pressure because the system is open. If you restrict the system with a smaller opening, you can bring the pressure up, but the flow drops also. If you restrict the intake, the pump output will be less, and at some point will starve the pump and it will cavitate.
But this isn't an open system, it's a closed system. Everything that goes out the pump comes back through it. Unless there is a restriction in the system the pump can't achieve pressure, electric or engine driven. But there are restrictions.
To think of the cooling system as just a pressurized container for coolant is misleading. In a simple container, pressure is equal in all directions. Cooling system pressure, however, is average pressure, usually measured at the radiator return tank. Actual pressures vary throughout the system. Coolant circulation is vital to heat transfer and temperature control.
Pressures are highest at the water pump outlet and in the water jackets near the combustion chambers. System pressure is lowest at the water pump inlet. Pressure is generated by the water pump and controlled by variable restrictions, like the thermostat and valves in the radiator cap. The system also has fixed restrictions, such as orifices in the water jackets and passages in the radiator.
If you just sealed the system and ran it, the system would continue to create pressure until something let go. Using a radiator cap that bleeds pressure off at a certain pressure keeps that from happening.
The real pressure in the system comes from the heat of the engine. The system is pressurized to raise the boiling point of the water above 212 F. Raise the pressure to 15 psi and the boiling point goes up to 250 F. Add radiator coolant, like ethylene glycol and 15 psi gets you 265 F.
As the engine creates heat, the water flow and pressure have to remove enough heat to avoid steam. Steam will allow the spot temperature at that point to skyrocket. Air in the system will allow steam pockets even if everything else works just fine. The highest point in the system is where air will go, and you have to have a bleed there. High spots in the circulation path will also collect air. Avoid them if possible, if not, realize that you are going to run with some air in the system.
I can't see where pre-pressurizing will gain you anything. The system pressure is zero when you fire it up. The heat load is low because you aren't generating much heat at idle. The water pressure is coming up, but even at a low pressure, there isn't much heat because you have to get the engine up to temperature. As you accelerate in 1st gear or at some point in the curve, the engine will be near to as hot is it's going to get. During that temperature rise, the pressure in the system is rising along with it.
I don't see anything wrong with pre-pressurizing either. You start with a system that has a 265 F boiling point. But I don't see that it would make a difference if it is an electric pump or engine driven. Only if the electric pump is under the flow capacity of the engine at some point, and I wouldn't run something like that.
The amount of flow has to match the ability of the radiator to remove heat. All the flow in the world won't help if the radiator won't handle the load.