So apparently my seemingly endless intermittent issue with starting is simply a degraded/faulty ignition coil. Which was not picked up by multiple mechanics and a nearly two month stint at a dealership with supposedly the best diagnostics hardware. Just glad I seem to have a diagnosis. Eventually got picked up after it went to a dealership who dismissed it as immobilizer (which was the original run-around with no solution), then to an auto electrician who then sent it to an independent ex-GM tech who diagnosed, and now I need to go back to the original dealer to get an OEM replacement coil (small town, 3rd-world country problems). Hopefully resolved soon.