First let me double check some of what you write.
(1) When you turn the key "on", the voltage at the injector jumps from 0 to 2.5V, and then drops back to 0 after about 2 seconds. Correct?
If so, then you either have a resistive short or a slow response on your multimeter. I'll assume a resistive short. Let's find it.
(2) Let's check for good voltage into your ASD. First, temporarily remove your ASD relay. With Key "on", probe at test point #4 in the original diagram. I am assuming you have 12V on both sides of Fuse 18, correct? Now assuming you do, then clean the contacts of your ASD relay and reinsert.
(3) Disconnect the two wire plug going into the coil. Did the injector voltage ramp up to 12V with key "on"? If so, your coil is bad. If not, leave it unplugged.
(4) Disconnect the plugs to your two O2 sensors, one at a time. Each time, check the voltage to the injectors. Did the voltage increase from up to 12V when key is "on". If so, you have a bad O2 heater in one of your O2 sensors. If not, leave them unplugged.
(5) Disconnect all of your injector wires, but make sure they are labeled or identifiable because you don't want to mix them up. When you do this, does your injector voltage ramp up to 12V with the key "on"?