Not Getting 12 Volts at Coil

Ouch is right! I hope I am not wasting bandwidth here as I don't actually know how the interlock circuit is wired, but as tenatious as you were at the rest of the wiring I would suspect that a schematic and your multi-meter could "Verify" the circuit and components of the interlock are correct.

At it's heart there must be a relay of some sort that grounds with the buckle engaged but I don't remember the unbuckle-rebuckle being required, just that the buckle needed to completed the relay ground. I would guess that the positive side of the circuit kills the power to the ECU so it likely does connect to it directly in some way thus the warning to leave unplugged (or fix it to be correct) is advised.

Its a shame this was so expensive but hopefully once you get some road time in your car the memory of lost $$ will fade quickly.