As you found a 9V battery is a preferred method to open an injector.
The injectors get 12V from the main relay and then are grounded by the ECU to fire them.
The injectors have a last chance filter in them, this can be checked once removed from the fuel rail. The filter is a tiny basket slid in place from the top.

You can swap the injector positions around to see if the dead cylinder follows, damn lot of work on these 6 cylinders.

You can also backflush the injectors, rig up something to spray cleaner into the outlet and open the injector with the 9V battery. Let the cleaner blast through.