Instead of second (or third or fourth) opinions from 'mechanics', get a second one from another gauge. They can and do go bad, especially the senders on
electric ones...
Not sure what a 'check valve' on an oil
pump is, unless they mean pressure relief valve, which, if stuck 'shut', could yield anomalously high readings.
Seafoam might help unstick one, as might other
engine flushes, or even just running it. Don't think I've ever seen mechanical problems from high oil pressure, but it could cause
leaks in coolers or sketchy hoses.