I think of borderlines like a three year old. They need attention, and they are going to get it, whether through positive or negative means. If it means getting cozy with staff (positive to them, bad for us), then they'll do it. But most often, staff are aware of their 'game' and don't participate, forcing them (in their mind) to act out.
When I deal with borderlines, I set specific limits on the amount of time that I spend with them. If I need to interact with them, I estimate how long it should take, and I have no qualms about leaving the room at the specified time. Borderlines have no limits, are not interested in what you have to say, and often get sidetracked because they are mired down in the problems that they are ruminating on in their heads on a daily basis.
Borderlines will monopolize every conversation that you have with them if you let them. I call their behaviors to their attention. "I understand Ms. X that you are talking about your childhood. But "MY" question to you was if you are feeling any physical pain today?"
You cannot use rules of polite conversation with borderlines. For instance, you cannot be polite and wait until they are done speaking so that you can ask them a question because they often don't want to stop fixating on their problems. I talk over them (politely) and repeat what I have to say or my question until they hear me. If I am being ignored, I will state that I will come back at a different time when we can have a two-way conversation, and leave the room.
Most importantly, the entire staff needs to agree and be cohesive when working with a borderline patient. If I was on the unit you described, I would escort that patient back to their room, explain to them that under no circumstances are any patients allowed to talk loudly at the nurses station as it is upsetting to other patients and staff. I would instruct the patient that when they are able to act appropriately at the nurses station, by using a quiet voice and not hanging by the desk all day, they are welcome to come over to ask a question or address a need. Then, ALL the staff need to back the primary nurse and follow up the same way with said patient.
If your staff are not doing this, then they are letting the milieu run them instead of overseeing the milieu. And, as you have seen, it is very difficult and upsetting to work in the former environment.