2000 Chevy Express P0137 and P0452 Fault Codes: EVAP Sensor, Fuel Pressure, and Running Rich Issues

My 2000 Chevrolet Express is displaying fault codes P0137 and P0459. The P0452 code indicates a low voltage signal from the evaporative (EVAP) pressure sensor located in the fuel pump assembly. Although I haven't measured fuel pressure yet, I suspect the EVAP sensor may be faulty—especially since I replaced both the fuel pump and sensor last fall when the pump failed. I'm aware that many 2000 Chevy Express models have known wiring harness issues with the fuel pump, so I'm wondering if a wiring problem could be causing this reading. The vehicle is running extremely rich, which is evident by the black exhaust pipe tip and poor fuel economy. The P0137 code refers to the downstream oxygen (O2) sensor, which typically indicates a lean condition after the catalytic converter—but my symptoms suggest a rich mixture, creating a contradiction in the readings. The van runs poorly when cold, with noticeable stalling and reduced power before the O2 sensors are expected to influence fuel trim. I also know that the fuel pressure regulator is prone to failure on this model. Additionally, during the fuel pump replacement, I accidentally bent the silver flap inside the fuel filler neck—now it doesn't seal properly. This could potentially allow vapor leakage, leading to EVAP system malfunctions and triggering P0452. I'm seeking advice on whether the EVAP sensor is faulty, if wiring or the fuel pressure regulator is at fault, or if the bent filler flap is contributing to these symptoms.