What was the purpose of the "additional storage area" found in Rosemary's apartment? Was this the link to Roman and Minnie's apartment where they could perform the Satanic rituals? Could Terry have been an instrument which needed elimination before she fed Rosemary information about the neighbors being practicing witches?
The storage area was a closet, resulting from the conversion of Minnie and Roman's huge apartment into two different units. This had been a normal doorway between the two sections of the apartment, until it was sectioned off. Much like dividing doors between two adjoining rooms in a hotel. Shelves where added to make some use of the space. But eventually, the space was used to travel between the two units, and it is through this portal that Rosemary was carried when she ate the 'chocolate mousse' which Minnie made.
Rosemary was inducted into the ritual because when Roman told Terry about what was happening, Terry committed suicide, jumping out the window. This is why Minnie yells at Roman, which Rosemary hears through the wall when she's in bed, dreaming, and we see the nun speaking in her dream 'I told you not to tell her. If you hadn't, we wouldn't have to do this!' Minnie is chewing out Roman for telling Terry, and blaming him for the suicide, which is correct. So now they have to plan to capture Rosemary and use her as a host for the new child.