There is no inertia switch on domestic GM vehicles. Those vehicles which have them (specifically Fords) interrupt the fuel pump circuit with the switch, not the ignition, AFAIK.
Even though you have 12V at the pink wire to the ICM (the ICM MUST have 12V on the pink wire from the 15A ELEC/IGN fuse, regardless of power on any other wire), verify the integrity of the connection at the ICM itself. There are aftermarket replacement connectors available for this connection because they are subject to corrosion and failure.
Verify a good ground on the base of the ignition coil module. I've had a few of those that would not run with corrosion between the mounting plate and ICM base.
If power and ground are present, verify both the cam position sensor (CMP) signal at the ICM, and the both crank position sensor (CKP) signals at the ICM. The CMP should connect via the brown/white wire. The CKP connects via the yellow wire and light blue/white wire for both low- and high-resolution signals. Power and ground for both sensors are supplied via the red/black and white/black wires, respectively.
To my knowledge, if the ICM has power and ground, and the CMP and CKP signals are present, it should produce spark regardless of the PCM. The PCM can control the spark advance of the ICM via the tan/black wire, and senses the engine RPM via the white wire, but should not inhibit ignition. If the PCM and/or wire harness are grounded, however, I'm not sure whether that would kill ignition or not.