It seems you have some underlying rich running condition going on there, and you have checked most of the common things that can cause a rich mixture. Your fuel pressure while running is right on spec, so it is probably not a fuel delivery issue. Since the pressure loss after shut down is back through the supply line you probably do not have any leaking injectors causing specific cylinders to flood with fuel. The pressure loss IS something you should be concerned about, since it probably indicates a badly worn fuel pump, but I do not think it is related to your present concern.
On these engines, the 2 things that come to mind that could be causing a long term rich condition are:
(1) Coolant temperature problem. If the thermostat is out of calibration and not keeping the engine temperature up to 195F the computer will respond by increasing injector pulse width because it thinks the engine is still warming up. Likewise, a coolant temperature sensor that is out of calibration, a low coolant level in the engine, or high resistance in the CTS electrical circuit can all result in the computer commanding a richer than normal fuel mixture.
Try measuring engine coolant temperature directly with a thermometer, and compare it to your scan tool's temperature reading to make sure they agree and that coolant temp is above 195 when the engine is warmed up.
(2) The exhaust manifolds on these engines commonly develop cracks as they get older; this can allow air to enter the exhaust stream ahead of the oxygen sensors. The extra oxygen in the exhaust then will be picked up by the front O2 sensor as a lean condition, causing the ECM to increase injector pulsewidth to bring exhaust oxygen back into normal range. So what happens is that the O2 sensor ends up reporting a normal fuel mixture, when it is actually too rich. Carefully check for ANY tiny exhaust leaks upstream from the front O2 sensor; a smoke generator tool works well for this.