So I thought I'd update this thread with my testing and results.
I managed to work out a method to fire the injectors with the fuel rail installed (intake plenum off) and the fuel pump at full pressure. The truck has a factory connection on the firewall to test the pump by hooking it up directly to the battery. I used a pressure gauge to see full fuel pressure.
I then used the 9v battery with a momentary switch to activate the injector through the harness. With my borescope I could look down into the plenum intake to see gasoline spraying. I am getting full spray pattern, proved put by testing an adjacent injector.
I could also see the valve for the #4 was partially open. Using my air gun I sprayed into the intake with the spark plug removed and water poured out. So my attempt at sealing the head gasket has failed and I can only surmise coolant is extinguishing the spark.
Still trying to decide what to do. The long process of again removing the head is not something I think I want to do.
92 Montero LS 3.0L V6 Auto, Stock, Original owner, 185,800K miles