Yes, indeed is a tough one.
The fuel pressure regulator - I tested it as unscrewing the return line from it and no fuel was coming out.I guess it it OK?
One of the fuel pump is OEM Bosch , the other one is aftermarket AIRTEX brand.
The system pressure measured at the FD is 92 psi and 86psi lower chamber.

The only thing I am not if it works 100% is the used FD...but how can I really test it?