My take on it is -
Your parents are overstepping Bs boundaries and always asking her about it. She doesn't want to tell them and is getting cross that they won't respect that. That's why they're tiptoeing on eggshells round her as if she's the problem, because they think they're entitled to information about her, so they think her not telling them is the problem and they can't see how rude they're being always asking and that she's justified in being cross with them. I'm guessing it's this because my parents do the exact same thing.
Since they can't get information out of B they ask you to be their flying monkey and obtain it instead. This part you can control. Tell them no. If you do have information about B, refuse to discuss it with or pass it onto, your parents. They have no rights to know anything she doesn't want to tell them.
They don't have rights to use you as flying monkey either. If they get cross with you for refusing to do what they want in obtaining and passing on information, recognise their rudeness in that and respond to them as you would to any other rude person, which might include choosing not to hang out with them if they keep being rude. Just because you share DNA doesn't mean they're entitled to your company. If they keep stressing you, don't spend time with them.
You've been brought up to gossip and to think it's normal and ok. It isn't. You can change that by refusing to participate. If you must discuss your thoughts about B with anyone, make it your DH. Don't discuss her with other family members. If other family members try to drag you into gossiping, respond only with neutral comments like "she's an adult, I'm sure she'll work it out". Don't give a judgement or personal opinion, keep those thoughts to yourself. If you're inclined to gossip in general, ask other non-gossipy friends to point out if they notice you doing it. That way you can start to recognise it yourself and re-train yourself to be nicer. You can tell people it's your new year's resolution.
You're going to have to be really blunt with B, even if it pisses her off. If you tell her you don't want to talk about something and she keeps on, literally tell her to shut up.
Yes, it's rude, but so is she being rude by not respecting your wishes. Rude people often don't understand or pick up on subtlety, or if they do they ignore it because they want to carry on as they are. It's not at all subtle saying you don't want to hear about something, so if she's ignoring even that level of directness, you're justified in being incredibly blunt to the point of rudeness.
Don't feel guilty about it and if she gets cross with you and starts having a go at you, don't tolerate that either. If she doesn't want to face rudeness, she shouldn't dish it out. If she respected your wishes not to hear about it, you wouldn't have had to resort to rudeness to get her to shut up.
Tell B you're really busy at the moment and can she please text or call first if she wants to come over. If she shows up without doing this, it's ok to not answer the door. If she calls or texts first or gives you a chance to say if it's convenient for her to drop by or not. You don't have to give her access to your home just because she doesn't want to hang out in hers. It's not as if she's trying to escape domestic abuse.
Not sure you're asking for opinions on B or not but I do think she's right to leave the relationship because she wants DC and her husband obviously doesn't like them. IMO he has been stringing her alone with false hope that he might change his mind, hoping to do that until her fertility runs out so then she thinks there's no point in leaving him because her chance at motherhood is over anyway. Meaning he gets what he wants: to keep their relationship but not have DC, it's a mean approach that doesn't consider Bs happiness in life. No reason she should put up with that.
If she's got BPD she should get help before she fucks up any DC she has by neglecting them emotionally. If she can't even cope with her own emotions as a single woman, it'll be a whole lot harder looking after herself as a mother and being responsible for DC's emotional/physical wellbeing too. So seeing atl therapist to work through her situation is a good idea. It's not your job to make her do it though.
I'd run a mile from the creepy choking guy myself but then I'd never be on a kink dating site either, so goodness knows what B is into. I don't like discussing my own or other people's sex lives though, so I'd be same as you not wanting to hear it.
You're not a bad person for not wanting all the gory (and worrying) details. The guilt is because you've been brought up that it's your job to fix things for everyone, so refusing to do it is going against your training and making you feel bad. It's not your job though and it never was, so the guilt is misplaced and best to be ignored.