Update: it turned out to be #2 above--sort of. I removed the Pressure Relief Valve and plugged that hole using the (probably not defective) original Pressure Switch. This gave capability to run the compressor to as high a Tank Pressure as I wanted (until explosion--theoretically). I decided that I would only run to about 170psi. As I ran the compressor above 150 psi the new/replacement Pressure Switch shut off the motor at about 152psi. Therefore the problem was the Relief Valve releasing at 150psi when it was spec'd to release at 160psi. So I installed a new Relief Valve (165psi was closest I could find) and now the unit is functioning properly.

Many thanks to SKATINGTOOLMAN for advising how to remove the Pressure Switch--without that help I would have gotten nowhere.