Lift pump design just basically idles once maximum output pressure has been obtained. So no damage to the lift pump will result. Test basically is to see if lift pump is still putting out a reasonable high pressure.

I did not look carefully at your gauge but assume it is liquid filled type. You are not looking for a peak pressure but as stable an average reading as possible.

A real restriction in the gauge feed line may moderate the reading average. Or needle bounce if you will. A gauge not liquid dampened is almost useless without some work.

Anyways closing off the return line will not damage the lift pump. It is the only way I can think of to see if the valves etc in the lift pump are still pretty good. Weak return lines on the injectors may leak though has been mentioned a few times during this test.

With a closed off return I personally think 25 pounds or more should be present then on the turbo lift pump. Much higher have been mentioned by some.

Idle is usually improved at higher pressure than existed. The reason is the power balance of the engine is improved. That is why the seat of the pants feeling of it is there.

A power balanced engine produces more power has to be a given. I have no ideal without thinking about it of why your idle is different. Other than better fuel pressure tends to on average put the timing forward a little I believe. The usual effect is a smoother idle and a quieter engine on the highway. I have always visualized 19 pounds as the sensible upper limit for fuel pressure. Those pressures in the manual are the absolute lowest allowable in my opinion. Still good information.