I hope I'm not covering this again, since I get the list in digest form.

My amateur garage would be removing the fuel pump relay and installing a
jumper wire between terminals 87 and 30 (the perpendicular center ones).
Keep in mind this will run the fuel pump from the second it is installed
and should only be inserted when you are planning to run the engine. Remove
it when the engine is off to prevent battery drain and flooding.

By removing the relay and replacing with a jumper the relay is removed as a
potential cause of your woes.

It is possible for a VW fuel pump relay to limit RPM. Not sure how it
works, but certain CIS injected Golfs and Foxes circa '85 - '87 had a FP
relay that cut off the fun at 6000 RPM. You could have a defective one of
those installed or some sort of industrial engine one perhaps?

I think looking at the distributor next as you have been doing makes sense.

Hope this helps.

Alex