The temp gauge is driven by a different sensor than the one the ECM/PCM uses, so the dash gauge working doesn't mean the ECM is getting the proper temp. In fact, the Code 15 blatantly states that it is not receiving any signal whatsoever from the CTS, and has gone into a fallback mode. I'm not certain about the L98, but if it's anything like the later LT1, the fallback mode for complete CTS failure is...sadly, nothing other than throwing the code and, on an automatic, applying the TCC early. You should find the FSM for your '90 5.7 (L98?) and check the diagnostic page for DTC 15 to be sure. I only have the 1994-1995 5.7 (LT1) FSM, so that's all I can go by. But if in 1994-1995 the only thing that happens is throwing a code and applying TCC early, I would assume 1990 would be exactly the same. Doesn't make sense for GM to do something more advanced on a far more primitive computer system.

However, you are correct that a failed-low-temp CTS should not cause the engine to not run at all. It will cause it to run pig rich, and will cause it to never enter closed loop operation, but it should still run. If it does not run, something else is going on.

I agree that a total cutout sounds like ignition. Check all connections between distributor, coil, ICM, and ECM thoroughly. Again, the FSM can actually help you with the kinds of readings you can get with your multimeter to check each of these components. Or you could just throw parts at it, like replacing the ICM. But 100% start with the wiring since you already know that was a problem before. And if that CTS is any indication, it sounds like your wiring in general is suspect. Perhaps a rodent decided to have a snack. Best of luck.