I guess I qualify for old school knowledge. The old compressors had a sump to store oil to keep the compressor lubricated. New, R134a systems, rely on the oil moving thru the system. The pressure relief valve popped due to high pressure, not overheating. If the cooling fan isn't running, the pressure cannot be reduced as it travels thru the condenser by removing the heat from the Freon. There is a possibility that the AZ charge up kit did have oil in it and there was too much oil in the system. But your high side reading of 100psi is lower than I would have expected to see. It is not uncommon for the system to not quickly stabilize after shut down. Buy messing with the pressure switch, you may have fixed a intermittent problem there.
The thing to question is, why was it low? A/C is a closed, sealed system that should not require service unless you have a leak. Look around for an oil stain on the A/C components. Find oil accumulating on the outside, you probably found your leak.