Update
Yesterday, I took apart the bank 1 intake to inspect the throttle body valve. Everything looked fine, and it was moving freely. Just to be sure, I wiped the surfaces clean with some throttle body cleaner. I followed the P2101 troubleshooting procedure starting on EC-419, and didn't find any issues. Once I had everything back together, I erased pending codes and ran the usual trio of throttle/accelerator/idle relearning. No new codes popped up this time.

Unfortunately, there are 4 permanent codes stored on the ECU now, including P2101 and P0300. I learned from reading a TSB that permanent codes (P-DTC) need to be cleared by going through specific drive cycles, so will need to work through that. It's an excuse to drive the Z more, I guess.

I will still try listening to the injectors, and will take a look in the fuel tank once the level drops lower, and then report back.