I haven't read the whole thread but have read most of OP's posts from the first and last pages.
I have ADHD too, OP, and I think you're falling into a trap which is not ADHD specific, but I think ADHD parents are especially prone to - of feeling like you need to persuade your child to accept your decision otherwise you're being unfair.
In fact, she does not need to accept your decision, because this is a boundary you have full control over. She doesn't need to be happy about it. She is allowed to be disappointed, annoyed, angry, upset, distraught, enraged about your decision. She does not need to understand it, agree with it, or anything. Obviously it's nice when those things happen - but it's not a necessary part of setting a boundary, and I'd argue that a lot of the time, as children get older and there are fewer things you draw hard boundaries around anyway, it's normal for there to be a gap where first they have to experience and move through the disappointment, and only later will they understand reasons and possibly agree. (Actually that's probably always the case even with toddlers, isn't it?) Trying to short-circuit this doesn't help, and in a worst case scenario, you're just reiterating that difficult feelings are dangerous and intolerable which is not a very good place to get into, as it feeds anxiety and feelings that a YP can't cope.
I saw the post about co-regulating as well and I get this as a desire - but co-regulation, while it's important, is really only one part of developing self-regulation. Another really important part of developing self-regulation skills is having the opportunity to experience emotional discomfort, not have adults panicking about it, and experience the fact that they don't last forever and you don't always need help for them to go away, they do fade on their own given time. IME, particularly with the emotional dysregulation side of ADHD combined with general emotional mess of puberty, it's even more important to have time and space to experience emotions without a parent getting overly involved or wanting to rescue you from them. It's just a balance thing and it's difficult. I find ADHD makes me very all or nothing. It is hard to strike a balance with sometimes holding back, sometimes engaging, sometimes pushing. Actually this is probably the thing I've found the hardest with the teenage years. DS1 is 17 now, I was diagnosed with ADHD when he was 10, I had more than half of my parenting him with no idea that I had this and then I started to understand it but it still takes time and I'm a lot more aware of my own patterns now than I was when I was newly diagnosed. And I'm probably still making mistakes left right and centre.
The thing is, you've been asked, you've given the answer, and now all that is left to do is to stick to it. Something like this is arguably easier than a pet, because eventually the event will come and go, and she won't have gone to it. She can't buy a ticket on her own and she can't get herself there, so that's the end of it. That is the boundary - you will not buy the ticket or take her (or give permission for someone else to). It's really important IME to recognise that THIS is the boundary - what you do / don't do (in terms of buying the ticket etc) or what you allow/don't allow (in terms of someone else buying her a ticket, or her bringing a pet home from somewhere else, for example). And this is not up for negotiation, so it doesn't matter what argument she puts forward or what bargain she makes or how upset she is about it. I know that might seem harsh, but you also know intellectually, don't you, that missing this event is neither unreasonable, nor will it harm her. You've made that decision and it's OK to do that as parents, in fact it's good parenting.
I think because my ADHD makes it tricky to do balance well, I do better when I have clear rules for myself and one rule it sounds like it would help to have is to establish when you/dad are pulling rank and drawing a boundary (non-negotiable) and when something is up for negotiation. Even if this is just a distinction you make privately or to yourself. Only negotiable issues (might) need everyone to agree, or should wiggle in order to increase the satisfaction of everyone involved. You might lean towards having a lot of negotiables in a ND household, although not everything will be a negotiable issue. However, if you're approaching a boundary with the aim of getting everyone to agree, then you're making it negotiable - which probably isn't what you want, which is why you're getting tied up in knots about it, because you've basically given yourself two conflicting goals. But don't worry, 12 is a very good age to realise this, because at 12 you're still a good few years off the point where you start to lose the ability to set certain boundaries at all because she will have access to her own bank/ID/transport/unrestricted internet. That's OK, but it is much easier to practice setting boundaries and finding the difference between a boundary vs a negotiable decision while you still do have this control.
I could get into the chores/animal care side but I think this post is long enough. But seriously. Lots of blind spots for parents with ADHD, I think. I could write a book about it (I have genuinely thought about doing this...)