Am I correct to assume that in post 1 you are actually trying to quantify the true vacuum pressure by reading the tiny increments on those gages? Engineers like me are always amazed how the public thinks sensor readings are exact, especially when presented digitally. There is always a +/- error band. In your car, I am pretty sure the 2 gages would see the same pressure when the compressor is not turning and you give time for pressures to equilibrate. The compressor does not make a tight seal from suction to discharge, and the expansion valve is also a metal-metal closure that leaks thru a bit.

Post #4 mentions 300 psig on the high-side causing slight weeping from a hose crimp. That might be a useful "relief valve". I would never want to see 300 psig since you are approaching the point where the compressor stalls as the clutch starts slipping and melts. I had that happen in both my 300D and 2002 T&C van, but we do enjoy >110 F days here that push AC systems over the edge.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans