During the oil service at the Chevy service shop, they did some further diagnosis specific to my install job on the EVAP purge valve. Turns out the ECM isn’t sending any commands to the Purge Valve, and the EGR is operational - it’s receiving commands and actuating.
The likeliest source of the problem is the ECM. I’ve already lined up a purchase of a used ECM which belonged to the donor car from which my car’s engine came from, and will be attempting to solve this on my own this weekend.
Cross my fingers and hope for the best.