First of all, where is the location of Oil Pressure scwitch? Could bad oil pressure switch cause my Bonneville to stall while coasting to stop?

What is the theory behind the relationship of Oil Pressure switch causing Fuel Pump to be inoperative?

The oil pressure switch/sender should be located just above the oil filter.

The Fuel Pump Relay is activated by the PCM for 2 seconds after the ignition is turned on and during cranking. Once normal oil pressure is reached, the switch part of the oil pressure switch/sender takes over providing voltage directly to the fuel pump. If oil pressure drops below a certain pressure, the switch opens and no voltage is sent to the fuel pump. This condition is usually accompanied by the oil light illuminating or low pressure indicated on the gauge.

You've been fighting this stalling problem for a while now, haven't you? You might want to get your hands on a scantool that can display real time data so you don't have to keep guessing.