Engine Problem - Please Confirm Diagnosis

[JEDD]

I am turning my car into a Boxster Spec/BSR. I have started the car routinely during the process of removing weight. However, just recently, after I cut some wires that led to the amplifier, the car stopped turning over altogether. There is a slight tick near the front when I turn the key, but not much else. I believe the problem is the immobilizer but I want to check with the community to make sure I haven't missed anything. Following is what I have done so far.

I reconnected the wires I cut. This didn't change things.

I checked the battery. My multimeter registers about 12.5 volts. Nonetheless, I put it on a trickle charger for a couple days. Car still won't start.

I put the multimeter on the positive battery cable where it connects to the front of the car. It registers 12.5 volts.

I checked the clutch switch. It looks fine. I jumpered the wires. Still won't start.

I put the multimeter on the battery and watched it while I turned the key to see if there is any drop off. I have read that a significant drop off would indicate a problem with the starter and/or starter solenoid. There multimeter read roughly 12.5 volts every time I turned the key and changed very little.

I pulled the number 7 relay in the rear trunk and put a jumper wire between the two large wires. I turned the key and the car turned over but did not start. I have read that this indicates there is no problem in the battery-to-starter chain and, rather, that the problem is with the immobilizer and/or ECU.

I have not yet attached the multimeter to the solenoid or otherwise checked the starter. However, I understand these issues are ruled out by some of the other tests I did do.

In retrospect, I (stupidly) forgot to disconnect the battery before cutting the few wires mentioned above (to the amplifier). I wonder if this may have triggered something in the immobilizer? The immobilizer looks fine on the outside and inside, but I know these things are very sensitive.

Any thoughts or suggestions would be greatly appreciated.