Fuel pump question....
So if the engine isn’t turning and the pump arm is on the back side of eccentric can the electric pump still push fuel past the mechanical pump?
Normally, with a mechanical diaphragm pump, yes.
A mechanical pump uses two spring loaded (poppet?) valves, one for the inlet and one for the outlet. Suction from the diaphragm draws fuel into the pump, then the valve closes. On the opposite stroke of the diaphragm, pressure forces fuel through the outlet valve, after which it closes. Both springs are only lightly loaded, since they are easily actuated by the 5-6# of pressure a mechanical pump creates.
Pressure from an electric pump ahead of the mechanical easily opens each of those valves (on the inlet, downstream pressure acts the same as internal suction), creating a flow-through.
For momentary "priming" use, a regulator on the electric pump should not be necessary, as most of the electrics I mentioned are pretty low pressure to begin with.
The issue, as I mentioned before, is whether the electric is a type that will allow "draw through" by the mechanical pump (in much the same manner as described above) during normal operation.
Easy test: Blow (or use low-pressure compressed air) into the inlet of the electric pump- if it flows air, it will work. If not, it won't.