You're not feeling over-sensitive, no. And it sounds a very toxic situation.
But: people clearly told you she'd been talking about you behind your back. And you know what the workplace adage about that is. How come she felt comfortable enough with them to bitch about you?
Your backstory about her mentions you and others noticing her crankiness. So you've all talked about her among yourselves in the past.
On the talking behind backs thing, you're probably quits.
She "aggressively interrogated" you and you "politely answered" yet she made a complaint about you resulting in a disciplinary meeting. The SLT clearly felt there was no disciplinary to answer (probably because they have better things to do than deal with tea urn spats) but were you given anything in writing as to the nature of the accusation? Because you should have been. Minutes should have been taken and you should have been given a copy. Because it's all very well "you can go we just have to do this" (unprofessional and unacceptable for both you and your colleague) but she possibly has some kind of assurance from them that you have been given a disciplinary (justified or not) and you have nothing to say you were exonerated, because it sounds like it wasn't a proper meeting. Were you asked if you wanted union representation?
It sounds very much as though it was an informal meeting for form's sake, which, whether the accusations are unfounded or not, is unacceptable, for you as well as her. You have nothing in writing to say that a colleague made a serious accusation which was unfounded.
The SLT sound pretty useless at this point. It could all have gone like this:
She makes a complaint against you. They assure her they will act. You are given a disciplinary. They tell you it's fine, you aren't in trouble. They tell her you've been disciplined. She feels vindicated and "knows" that you were in the wrong. Or, they tell her that you were vindicated and she's in the wrong. Either way, neither of you has been treated correctly.
The later row- presumably connected to the initial one where you refused to tell her why you'd been given special (unpaid) leave (?) how do you know what she has or hasn't told SLT? Surely if the "verbal assault" is part of the first row with her, then that's exactly what she told them about? Or is she saying you verbally assaulted her at some other moment?
You immediately went to SLT. What did you say? What are they going to do?
Dreaded O? Ofsted? Hopefully they'll see what a useless and unprofessional SLT you have.
Is it bullying? No. I personally don't think it is. It's colleagues who don't get on, who clearly talk about each other to other colleagues and to SLT when they have disagreements.
On the basis of your version of events, no, you've done nothing wrong- but although you had no obligation to do so- if you'd told her your leave was going to be unpaid instead of refusing to tell her, maybe it wouldn't have gone this far.
Her version of the row was obviously very different to yours, or you'd never have had the non-, disciplinary disciplinary. They'd have told both of you to stop acting like the children(?) you're working with and get on with your jobs. Likewise, presumably they ascertained from her that the row was over your 2 days leave, and presumably told her that you were perfectly within your rights to take it, and other people's leave was nothing to do with her. (In the same way her past leave grievance was nothing to do with all the people who clearly talk about it)
I do agree you need to go back to SLT and insist on a formal meeting, with representation if necessary, minutes taken, and something in writing given at the end. Because if your colleague is hellbent on pursuing this, that's going to be her next move, seeing as she's still insisting on the verbal assault.
Good luck.