When putting voltmeter + on the other pin on the primary or secondary clip and voltmeter - on negative battery cable i get a slow and steady discharge from 5-6v down to 0, which makes sense since the voltage is discharging/getting used
This makes no sense, you should have constant 12.4 on each terminal key on, the ECU briefly grounds the injectors to fire them, so brief you’d never see it without an oscilloscope. Recheck to confirm. Also the injector resistors don’t alter voltage, they limit current.