Not sure if you mean the plunger is seized in the up position or not... Just in case:
When the engine is running there is fuel pressure pushing the plunger down, which pushes the plate up. When you shut it off some pressure remains for a while. If someone presses the plate down, the plunger will go up (and the plate will flop when you tap on it). If the plunger stays up and sits for a long time, it may seize in that position. It may be that the adjustment screw is OK.
Running the CO screw down (CW), makes the plunger go up and the A/F richer. As you suspected, if it is was stuck up, the engine would start to flood as soon as you turn the key on. Definitely a source for oil contamination (and washed out rings).
You could rough in the CO screw as follows:
Have fire extinguisher ready.
When everything else is together, put the injectors in a container.
Jump the RPM Relay and watch the injectors while someone turns the key to ON only. If they spray, turn the screw (CCW) just enough to make them stop when you remove the wrench. The CO screw will then be roughed in.
You are correct on the gold and anti-tamper plug.