You need someone who understands what is in each line to decide whether what is there is what I say. I can't tell you how to plug the lines as I don't know how they are constructed.
The system I describe is a by-pass to fuel return of an important control pressure. You do not want this to happen so removing it is the way to do it. Plugging it is a temp repair. Safety is of the essence!
Bosch Master, ASE Master, L1
33 years MB technician