Yes, that helps a lot, and that is most likely the problem.
It appears the ignition lock cylinder (GM part#12458190) contains the key pellet sensor, and electrical pigtail which connects to the BCM. It's about a $100 part and appears to be readily available.
I've never dug into this before, but since you seem to be good with electrical stuff, I believe you can bypass this sensor by unplugging that lock cyl connector (the one that contains just 2 white wires), and connecting a resistor of the same ohm value as your key pellet into the connector that leads to the BCM (leaving the switch pigtail disconnected). Might be worth trying before you drop $100+ on a new cylinder....If you have a Radio Shack or source for electronic resistors near you that is...
To measure your key pellet, just set your meter to ohms, and probe each side of the pellet in the key. There are 15 possible resistance values:
402 (ohms)
523
681
887
1.13k
1.47k
1.87k
2.37k
3.01k
3.74k
4.75k
6.04k
7.50k
9.53k
11.80k
You may need a combination of resistors in series to add up to the correct value needed to match your key. Solder the resistor(s) to some wires to create a jumper loop that plugs into the connector leading back to the BCM.
That should bypass it, and the car should then start and run. If it does, then you'll need to replace the ign lock cylinder to correctly fix it.