Well, you got a lot right and a lot wrong.
The line in question is the load signal. Since manifold vacuum works against atmoshperic pressure I guess it is altitude compensation at the same time.
Usually when the diaphram is effectually leaking one can remove the oil adding cap and put ones thumb over the hole and the engine will change running speed. If the leak in the diaphram isn't that big then it is inconsequential.
Bosch Master, ASE Master, L1
33 years MB technician