Open Relationship challenged by return of partner's toxic ex

TW: Suicide Ideation

I think my partner's toxic ex (who I hate) is trying to pull her back in under the guise of 'mutual healing'. My partner and I are in a committed open relationship where one of my boundaries is that she never sleeps with this particular(ly manipulative) ex because of their history and the ex's history of manipulation. The ex is poly, all of us are queer cis women.

Last weekend they had an emotionally charged accidental day-long discussion about their tumultuous relationship - which ended 3 years ago - at a festival they both attended separately. My partner was living with untreated BPD at the time of their relationship and she did really emotionally scar this person. Badly. However, this person also has a history of acting out, with suspected/undiagnosed BPD themselves and I've also experienced their manipulative and aggressive behaviour first hand (I met the ex for the first time a few months back, where she instantly started a fight with me and spent the rest of the evening being competitive and combative).

So the ex spent a whole day last weekend telling my partner about all the ways in which the relationship had made her very nearly commit suicide (even attempting recently over it), sink into alcoholism and more. My pwBPD took the whole day to listen, apologised from the core of her soul and took real accountability, though the conversation spun her out monumentally in the ways she's responded to the shame triggers around all the details of how she's badly affected this person.

Since that day last weekend, my pwBPD has had a huge mental health spiral - two days of uncontrollable crying followed by three days of a manic high - all aided and 'held in a truly healing way' by the toxic ex, whose house she's been staying at in the city the ex lives in (alarm bells). My partner's also been written off work, citing it (correctly) as a mental health crisis. It's been serious but it sounds like she's on the mend. However, the intuition alarm bell's going nuts. I'm out of the country, or else I'd be with her myself, but we've been speaking on the phone and I've asked her if she's slept with / kissed her ex (or similar) and she's said no, but that her ex spanked her on the ass once and she told her not to do it again. I believe that they haven't slept together and I don't care about the ass spank as an isolated incident. We have really good, open communication, and I can also tell if she's ever swerving the truth, so this isn't what's concerning me.

The concern is that the ex is playing the long-game and ultimately wants to be poly with my partner. The ex has said previously that the only problem in that relationship was my partner's BPD behaviours - which she's now getting under control. She's also tried multiple ways to get back into my ex's life after their break-up, which my ex shut off every time, but the new level of intimacy that's developed as a result of all of six days' intense trauma-bonding means that they've both basically agreed to be back in each others' lives.

Now, if this was just the ex back in the life of my partner as a friend in the periphery margins - who I didn't have to see - that'd be one thing. However, my partner had an issue with me on the phone earlier when I said how much I hated her ex - multiple times - and brought up the incident when I first met her. My partner responsed with words to the extent of: 'Well, you're going to have to get to like her because she's going to be around a lot. I've really damaged her, so I have a duty to help rebuild her life. Yeah, she was nasty to you when you first met, but I that behaviour came from a place of anger that she didn't have before she was with me, and it can just go down as one of those disastrous first meetings, but you'll get along eventually.'

So I'm furious at this response. Being told that someone I really dislike (I can't express to you all enough how irritating I find them) is now going to be part of our lives is so fucked. Where's my agency in this? Was I consulted? No! I was told I need need to change my persepctive on the person, and 'not to worry, they'll be on their best behaviour' (though it's their personality I have more of an issue with). I also forsee conversations unfolding along the lines of: 'Well if you don't want to be around the ex, you don't have to be, but we're already planning lots of things to do together, so I guess there'll just be a bit less time for us to do things togeher', and then - BOOM - suddently they're in a poly situationship. Now, if we'd agreed to be poly I understand it'd be up to my partner and my partner alone to choose who she's with, but we never agreed to poly - just casual open relationship ENM, and though she hasn't crossed any of our agreed boundaries with this recent thing, in the past she's adhered less strictly to our boundaries than I have (BPD impulse is tea), so when it comes to setting new boundaries around this, I want to get it right.

Where do you draw the line about making allowances for your partners with their exes, when you feel there could be a tangible risk? Do I shut it down / veto this new phase of friendship when I know she's looking at the situation as a way of making amends for her previous bad behaviour? Do I tell her it's her ex or me? My gut's screaming "YES and get that bitch out of here" - but maybe that's too rash.

Last things: it's worth mentioning that my partner's manic right now, so isn't in their usual mental state, but I really could do with some suggestions for how I can really clearly frame this to them that allows her her own personal freedom but doesn't compromise me in the process. We're also engaged and serious about it, so I could be overthinking, and if anyone thinks I have the wrong take on the sitatuion I'm very open to other persectives and takes too.

TYSM - Stressed In Montreal