Fuel regulator question

Not necessarily. The fuel pump may have weak output. Use a hose pinch-off pliers to pinch the fuel return line. That will force the fuel pump to hit the highest pressure it is capable of.

With the engine off/no vacuum, fuel pressure should be 50 - 60 psi.

I think you may be looking at this wrong. The goal is not to make fuel pressure go up when vacuum is removed from the regulator's port. The purpose is to make the pressure drop when vacuum goes up. Instead of removing the vacuum hose and expecting to see higher pressure, you need to connect a hand-operated vacuum pump, and expect to see pressure drop.

There are two forces acting on a molecule of fuel at the injector's pintle valve. Fuel pressure is pushing on it, and intake manifold vacuum is pulling on it. That vacuum goes way up during coasting, and that would cause an excessively-rich condition and increased emissions. Fuel pressure is lowered during coasting to offset that higher vacuum, so the combination of those two forces remains the same.

56 pounds of fuel pressure is perfect. Why are you working in this system? What problem or symptom are you trying to solve?

You found good fuel pressure so it's doubtful this is a pressure or volume issue. A better place to be looking is at fuel metering, meaning how much fuel the Engine Computer is requesting from the injectors. You need a scanner to view live sensor data and see what the computer is responding to. An Engine Performance specialist knows what to look for, but you can start by looking for anything that is obviously not right. In particular, look at temperature readings for intake air and coolant. If it's 30 degrees outside but the intake air temperature sensor is reading 70 degrees, for example, too little fuel will be going into the engine.

Another thing that can provide some clues are the "fuel trim" numbers. If you find "short-term fuel trim" (STFT) numbers are high positive, it means too little fuel is being calculated compared to the amounts programmed in at the factory as starting points. Those numbers should be fairly close to "0".

Another place to look on all car brands other than Chrysler products is the fresh air tube between the mass air flow sensor and the throttle body. There can't be any loose hose clamps, cracks, or other leaks in that hose, or vacuum leaks. If any air sneaks into the engine without going through that sensor, it won't be measured, and no fuel will be calculated to go with it. That can cause erratic idle speed, hesitation or stumbles, all the way up to a crank / no-start if it's bad enough.