[flatheadmurre]

The beauty of a mechanical pump is that the engine just loads it up to start position.
If the need for fuel is low no need for bypass or pressurerelief valves it just travels back a shorter distance before the pushrod puts it back in the starting position again.
And as stated the spring tension sets the pressure.
If you add an electrical pump before the mechanical producing a higher pressure it will compress the spring and stretch the diaphragm more then it was designed for, that might lead to a failure filling up the crankcase with fuel.