Sounds like youve done quite alot of digging and replacing already. Just wanted to throw some Ideas your way.

Im more fimiliar with the PSI system then Impco but i think alot of the system is similar.

When you had the impco diag software hooked up to do the injector kill test was the ecm recieving a crank, cam and oil pressure signal?

Continuity checks are ok but not great. Continuity can check out good even on a bad wire thats not capable of handling a load.

Id have to look at my schematic but pretty sure the power relay should supply power to your ignition and fuel system components.

On the PSI system when the key was turned on the ecm would cycle the power relay on for approximatly 2 seconds to do a check then turn it off again waiting for crank sensor signal and oil switch to confirm engine rotation before turning it back on again.

Have you tried bypassing the power relay with a jumper from terminal 30 to 87? Have you checked the relay terminals to see if your missing control ground from ecm or power input?

Have all ecm powers, grounds and key switch signals been checked not just for continuity but loaded voltage drop aswell?

When it was running and you said injector 4 had intermittent power did u confirm voltage was steady on the other 3 injectors? Or trace the wire back to the splice for the 4 injectors?

Sounds like you may even have multiple issues not just one. And like I said i do have info on these but havnt looked at one in a little while. There not very common around where I live.

If I think about it later when Im near my stuff ill take a look to see if I can think of anything else