Wow, scorpisosis, this is a really complex and difficult position. I totally understand why you're reluctant to go to HR due to the "harasser" making a calculated "friend" of a senior HR official. I would make sure all your ducks are in order before going that route. Reality is, you're probably going to have to go that route at some point if this woman feels confident that her tenure position is well protected.

I agree with the suggestion that you contact your harasser in written form, detailing her continual violation of your boundaries. I wouldn't do it by email, I'd do it with a registered letter. Yes, it will probably enrage her, but this lady's track record seems to indicate that she isn't getting the message and the only reason you're contacting her is to document her transgressions. This is for your benefit not hers. Keep an ongoing journal with detailed notes on any retaliation from her at work. Keep all her emails and/or texts. Allow her phone calls to go to voice mail so you can have a recording of her messages. She might get so angry about your refusal to engage with her that she might trip herself up by leaving angry voice messages and/or emails.

As someone else said, if she approaches you at work, calmly get up, excuse yourself and move to another location. I think it's great that your fellow colleague is willing to keep an eye out to come in and help out if she corners you. Unfortunately, you're the one who is going to have to remain cool, calm and collected through any encounters with her. As the junior staff member, you're going to be under closer observation. I would imagine that others in the department are well aware of this woman's wackiness, but they probably don't want to get involved. As someone else mentioned, the junior staff member is easier to sweep under the rug than the senior tenured member. Not fair but reality. However, if you document and keep your distance, you have a good chance of getting out of this situation intact. Don't allow her to push your buttons, although I totally get how the whole thing gives you the willies!

I don't think you're going to be able to keep her from "bad mouthing" you to others. But that kind of stuff happens at the workplace all the time and you know that. If you are well liked and respected by others in the workplace, you're a target by other petty individuals who resent your skills. Just hang in there and remain cool to her viciousness. I'm so sorry this has happened. Toads like this never seem to get their due because they are so skilled at manipulation.