Pressure, pressure, whose got the pressure:
The fuel pump produces it - maybe 6 bar/90 PSI? If it is weak enough, you might not be able to hit the system pressure target. Yours is new.
System pressure: the line from the pump, after going through the filter and accumulator, has pump pressure reduced by a regulator in the fuel distributor. This can be adjusted by adding or removing shim washers. Your shop sounds like it did this, though I don't know which way they went. Maybe added one, because your system pressure is too high. Getting it back into spec would be a good idea. If this were mine, I'd remove a washer. I've had a FD apart, have a bunch of books, etc. But it sounds like this would be a challenge for you. Not for a shop with CIS experience, though. System pressure gets sent to the WUR, and also to the fuel injectors.
Control pressure: this is set by the misnamed warm up regulator, which really is the control pressure regulator. Control pressure affects the relationship between the amount of air flowing into the intake ports and the amount of fuel injected. More air needs more fuel, etc. It operates counter intuitively - more pressure means less fuel.
The warm up part of the WUR reduces the control pressure while the engine is cold. When the engine gets going, the WUR pretty quickly increases control pressure. This is why you see the two figures, one for a cold engine, one for a hot.
Cold engines like a richer mixture for starting. That's what this is all about. However, if too rich, there are problems, just like being too lean.
You have measured the cold control pressure and the system pressure. What is the warm/hot pressure? And what is the 10 and 20 minute residual pressure? Easy to measure - you've got the gauge set up. You should do those before moving your gauges to check for return line obstructions. I'd put those way down on the list - pretty hard to have anything partially block a return line.
There are other adjustments here. There is an idle speed screw. If the cold idle is too low, you can increase or decrease the throttle bypass air to lean the mixture - or richen it. That isn't likely to help with the starting problem, where you have to put the pedal down a bit. But could with the idle. Of course, it might mess up the warm engine idle.
There is a screw which affects the relationship of intake air to fuel. When you take the air filter off, and reach into the air box and up on the right side, you can feel a lever. More air flow, the sensor plate moves the lever up, and this increases the fuel flow. A screw semi-buried down inside this side of things, reached from on top between the FD and the rubber boot through a small hole, adjusts the relationship. A shop sets this by using a CO meter, or nowadays more often an air fuel meter. There are techniques to do this at home, so to speak, also. Probably not where your starting problem is, but could be. It is used to get the emissions where they should be, but can be adjusted a bit rich for more power. Too much either way, and you get various problems.
Two additional checks have been mentioned.
--You can check for air leaks by having a smoke test done. If a shop asks what is that, try another shop. If you have air leaks, fix those. If you don't have leaks, one less thing to worry about. Air leaking in past where the air flow is measured messes up the air fuel ratio.
==The frequency valve system, which works with the oxygen sensor to fine tune the mixture so your air/fuel ratio is very close to 14.7 for complete combustion, is part of the equation. If it is disconnected you can experience hard starting. I learned this when putting a 3.0 into my 2.7. The 2.7 CIS didn't have this feature, and I figured I didn't need it, so didn't put it in. Once I hooked it all up, even though I didn't have an oxygen sensor yet, the car started much better.
The white circle is the frequency valve.
You can reach back there to make sure it is plugged in. With the engine running, you can reach it to feel with your fingers if it is vibrating or not. It should vibrate. You could remove the connector with the engine running to see if it feels different, and again when you reconnect it. If it vibrates, one less thing to worry about fixing your starting problems. If it doesn't vibrate, well that's a whole different line of troubleshooting.
Since the car runs great when warmed up, I don't think anything especially bad is going on that would be quite expensive to fix.
But give us the hot control pressures, and the residual pressure figures.
Tony knows these systems almost by heart, and differential diagnosis should work with enough pertinent information. He's a methodical engineer numbers guy, so give him the numbers.