One last quick check. Open return line at injection pump.Does fuel come out when you crank engine? If so there is enough fuel present in the pump under at least a little pressure to supply the number one element.

The other manageable possibilities as I think you have a more modern injection pump than the 123s use. The delivery valve is fouled and not seating by something.Could be a section of broken spring for example. Or the elements seals have failed letting the elements pressure as it builds up escape internally.

Other than those two things I expect it is beyond the average person to repair in the pump. So a good used pump is in order in my opinion at that point. At least they are available..