mamasin hope you're ok. You absolutely did the right, and brave, thing reporting the harassment and pursuing it when it wasn't being dealt with. It was the right thing to do for yourself and also for any other woman that has or could suffer similar actions from this bloke.
Unfortunately some people just don't like it when others 'rock the boat' for many reasons. I am sure HR kept your complaint confidential, they obviously realise the importance of doing that as they have made that point to you but it may be that the man you are complaining about said something and it leaked out that way. Presumably lots of people know about the initial incident anyway so there might be some guesswork.
Having said in a previous post I wouldn't have advised involving HR initially, on the other hand if they have made the point to you about it being kept confidential but you have reason to believe someone isn't doing so, then you are right to let them know, so from that point of view you have done the right thing contacting them. From your OP it sounded like you were hoping for some action from HR though to deal with your unfriendly and lets face it extremely childish ex-colleagues. I have huge sympathy, it can't be pleasant, but in the scheme of things, as these are not people you work with any more, I don't think any kind of complaint will either be effective or in anyone's interests including your own.
You mention in your OP that you haven't heard anything back from HR in response to your letter about your ex-colleagues' behaviour. Depending on how you phrased it they may well have assumed you were just letting them know there is a confidentiality issue somewhere rather than expecting any meaningful response from them.
If you have little or no contact with most of these people, I would be inclined to ignore them tbh, but as you still have regular contact with your old line manager I think you need to address her behaviour simply to make your own life a bit less unpleasant. I do think approaching her in a calm, mature way asking if you can have a discussion about your working relationship is the way to go, saying that you are concerned there is a problem and would like to get it out in the open and discuss it so you can work better together going forward. She is being very childish and a mature, reasoned approach will be difficult for her to ignore or respond badly to, and will highlight how ridiculous her own behaviour is.
I don't want you to take this the wrong way, but do be wary of turning into a 'victim'. You have genuinely been the victim of some nasty behaviour at work, and also of some incompetence, apathy and insensitive treatment following it, but there is sometimes a tendency to become a little bit immersed in 'victim' mode, rather than seeing each incident in your working life as it is. I hope you don't mind me saying this, I am doing so because I know you slightly, we've corresponded and I am involved in your situation. As always, when I am advising people, my focus is always on identifying what is the best outcome for them, and identifying the best way to achieve it, and that is what I want for you. If you do start going into automatic 'victim' mode (which is very common when people are involved in long drawn out situations like yours), it won't address any of the problems you may encounter effectively, and may make them worse. If you do try to involve HR when you feel someone is behaving badly, it will reflect badly on you. Those people you are complaining about will resent you and HR will not take you seriously when there is an issue that really does need their input and action.
I hope you are doing ok and making progress with your initial complaint. I am sorry if anything I am saying is not what you want to hear but I hope you also understand that I am trying to advise you in your own interests as I always do with anyone and that doesn't always involve 100% support for whatever someone I am helping says or does.
Do email me if you want a chat off-line