If there is little " through-put" from the return line, then the pump is weak. Your symptoms point to that. There is a pressure regulator in the fuel distributor ( push-valve) and also non return check valves in orifices that feed the four injector hoses. They don't "break" until about 40 psi as I recall.
So a weak pump just doesn't cut it. You need a new main pump.
If you really need to convince yourself then jumper the main pump and run it, lift the injectors out of the head, and lift the sensor plate. More than likely the four injectors will flow, but not all at once. One will start, then another, and the pattern will be less than conical and the injectors won't sing as they are supposed to make a high pitched whine when working right.