It simply makes no sense at all to me why a professional tech skilled in the ways of automotive air conditioning would only check the low side? This isn't really a good example, but to me it would sort of be like checking the tire pressure of the front/rear tires on the drivers side and not the passenger side.
I wouldn't give spit for one of those cheapy tire-gauge type a/c pressure testers. A/C pressures need to be checked with manifold gauges that are being handled by someone well-versed in their usage.
Based on what you've presented, I'd say you need to hire someone else to bring closure to this issue.
My 2 cents.