Just had an idea today... To diagnose the supposedly blocked vacuum line, try finding a piece of piano wire and snake it down the line from the top end near the check valve. This should reveal any obstructions and may clear them out if used in combination with a little compressed air shot in from the WG transducer side.

Do a search for egr bypass 606 turbo. I outline how to trick the egr on the turbo models. The computer moniters boost through the map sensor. I have plugged my wastegate transducer and I have a pressure wastegate with no cel's. I still have the transducer plugged into the wiring harness. If the map sensor is reading normal boost pressures you will have no cel.

From what I read in your thread is that the MAF should drop about .5 volts when the ERG opens. I am still wondering what the system looks for to trigger the wastegate solenoid code (P0243)
I am going to try swapping out the transducers to see what happens.
In the past I have always tried to have two vehicles that were the same just for the purpose of troubleshooting. At one time all three of my kids had 4 cylinder Mustangs. It was easy to figure out what was bad by swapping parts or comparing readings.
Now I just need to convince the wife that having a couple more E300s could save us money.

One more thing I am going to look at are the little filters that filter the exhaust end of the transducers. I am thinking if these are somewhat restricted the action of the wastegate or EGR could be sluggish. Perhaps the computer sees this and would give a CEL.

If I can't clear the blocked hose I will just leave the bypass hose installed as is since the system would be working as designed.

I would swap the transducers, but you know that already.

The ECU monitors current draw at the transducers and sets PO243 or PO400 if it isn't what it expects to see.

I beleive that by running a hose from the EGR transducer to the wastegate you proved the wastegate and the EGR transducer both operate, but did not prove the wastegate transducer works.

If the wastegate solenoid is fine and plugged in, then the only thing the computer is looking for is proper boost levels from the map sensor and maybe some values from the maf.

Today the car didn't produce a CEL even with full power. So for some reason everything is happy now. I will try to find why the other vacuum line is plugged but in the meantime I can leave the bypass line installed. It might become the permanent fix.
The last mystery is why don't I get a CEL even though there is a broken glow plug stuck in the head.