Back on track hopefully. I did not go back over your posts but have the valve settings ever been checked since you got the car? This just came to mind because of some milli volt variences I thought I noticed in your several test results. Sometimes they seemed good and other times bad. Changing valve stem lengths with thermal condition came to mind.

I am also getting more interested in making sure fuel pressure supply is good in the injection pump before going too far along. I think the fuel pressure is not the problem but can be contributing to any varying of the mili volts may be an outside highly unlikely remote possibility.

Closing off the relief valve line may change the milli volt readings if there is currently overflow from the relief valve. If no flow exists the fuel pressure may be too low anyways for good general performance.

For the time it takes not being certain of anything sometimes I do a tune up before really getting into the predominate issue. It is just my method of trying to reduce the everyday experience value I lack as a working mechanic a little. They develop a form on intuition instead that is priceless over the years.