Engine is idling rough and is running rich.

Lacking specialized pressure/vacuum guages, would the FPR be the most likely culprit for the lean running?

While that sounds like a contradiction, it is actually a very common situation. If the air leak is at only one or two cylinders, the oxygen sensor and computer will compensate forcing the remaining to run rich.

Check your plugs to see if it is running lean or rich, and whether all cylinders are alike. I think there's a page in every car manual with photos of plugs and what they indicate.

Pulling off the hose when it is running is just a bit dangerous and misleading anyway. If it had been leaking fuel, it would have sprayed on a running motor. The results can be misleading, because by removing the idle vacuum from the regulator the fuel pressure is increased (richening) but by leaving the hose disconnected, unmeasured air is allowed in, making it lean again, all the while, the computer is trying to correct it based on the oxygen sensor.

If it was my car, with no gas at the FPR nipple, I would move on to vacuum leaks and leave the FPR alone. But I quickly reach for the gauges when things don't seem to add up.
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Art Benstein near Baltimore

Give a man a fish and he will eat for a day. Teach him how to fish, and he will sit in a boat and drink beer all day.