Update:

The check engine light cleared itself!! Turned off yesterday as I was driving. I basically didn't do anything except that I put in a bottle of Lucas Fuel Injector Cleaner and let the gas tank run almost empty before filling up again. I didn't do any resets, didn't pull any breakers, didn't disconnect the battery, didn't replace the cat or the O2 sensors, etc. This time I made sure the cap was on there securely and heard a few clicks, because the person pumping the gas just threw it on without a single click. I drove it for about 50 miles before the CEL extinguished. I did step on the pedal like once or twice and had the throttle wide open for a few seconds while driving.