Yes, restricting airflow too much can damage your Advent A/C by causing coil freeze-up or compressor strain. The dividing wall is removed to eliminate air turbulence and improve ducted airflow distribution.
Your Ford 7.3 Godzilla oil pressure readings of 28-30 PSI at cruise RPM and 18-20 PSI at warm idle are within acceptable ranges. The key indicator is maintaining pressure above 10 PSI per 1000 RPM under load.
E-350 and E-450 use same V10 engine with nearly identical 8-10 MPG fuel economy. E-450 has heavier suspension, brakes, and higher GVWR. Choose based on weight capacity needs, not fuel economy.