1997 Honda Accord P0170/P0171/P0301 Codes After EGR Cleaning - Low MPG and O2 Sensor Readings

I own a 1997 Honda Accord with a 2.2L engine. After cleaning the EGR ports, I started receiving P0170, P0171, and P0301 diagnostic trouble codes. I've ruled out vacuum leaks—tested thoroughly using starter fluid and propane—and found no issues. The O2 sensors (Bosch) were replaced 14 months ago and are still functioning properly. The EGR valve operates correctly and stalls the engine when vacuum is applied. Fuel pressure and regulator readings are within normal range. I'm now experiencing extremely poor fuel economy—down from my usual 26–28 mpg to only 18–19 mpg. During highway driving, my live data shows the primary O2 sensor reading consistently at 0 volts while the secondary O2 sensor reads around 0.895 volts. This is highly inconsistent with normal operation. I've taken it to a local repair shop, who replaced the MAP sensor based on suspicion, but I suspect they're falling into the 'replace every sensor until it works' pattern. My current MAP sensor readings show 7–8 psi at idle and 28 psi at wide-open throttle (WOT), with normal variation during driving. Any insights or troubleshooting steps for P0170/P0171/P0301 on a 1997 Honda Accord? What could be causing the O2 sensor anomalies and the significant drop in fuel efficiency?