Just reading this and trying to catch up / process. Will try and think of a few things about this because I've seen this from a weird angle.
In terms of it being a cult, if that's what's going on, the leaving your job and trying to get involved isn't likely to help - if anything it will be a disaster. The problem with cults is they have an entrenchment issue - it's them and us so the more you push the more it entrenches.
The other thing is she is 18. That means joining the pta is good for a year at most and actually when people join a cult they are usually in it for several years. And it's like drugs and alcohol - the only person who can say no is the person at the centre of this and the only way they usually do that is by having an epiphany through a series of events (usually a crisis) or because they have significant life changes that make them question things. The one way to counter a cult isn't by saying no, it's by encouraging questions to niggle under the skin. The cognitive dissonance stuff to be picked at slowly - not in a barrage but on the terms of the person involved - it's about picking moments.
The one good thing with your daughter being 18 is that there's going to be some significant life changes going on in the next year or two. The instability is the cause of all this no doubt, but it's also probably the best thing in terms of her suddenly dropping out of the cult too.
Seriously though, quitting your job is a kneejerk reaction to something that's not going to get fixed anytime soon and you need to start seeing things this way
At some point I think you need to have a conversation about medicalisation and complications - I'd take the line about 'informed choice' and making sure she is - that's stuff like comordibities, long term consequences, lose of sexual function, infertility etc etc. And stressing that nothing is reversible as much as it's being pushed. You'd be better spending time preparing for this. You would need to stress it's her choice but you want her to make the right one. And actually make the point about medical supervision and not getting drugs off other people etc etc in case that's happening. Centre her heath and well being. Everything else if peripheral to that in the sense it's less of a priority.
I may post again later, but you need to understand the dynamics going forward. There is a halfway house between positive affirmation and outright rejection. That's the one you need to tred. I would be stressing stuff like she also needs to give you time to come to terms with things and to offer you reassurance (perhaps play we are the old gits, educate us card - to open up a narrative where you can act stupid and ask questions).
Given I no longer speak to my brother (not have the remotest desire to), I'm not necessarily an expert here (more to do with having a shit relationship with him to begin with). My mother's affirmative approach doesn't seem to have helped at all - I don't think my brother speaks to my mum much nor my dad at all these days. I think he was much further sucked in, very early on and I was always determined to reject us (one of the things he told us early on was that it wasn't uncommon to reject family - even if they were understanding - because it was too much of a reminder of their former selves etc so I don't think we could have done much differently to have a different outcome). He was in his mid to late twenties at this point and I think that's slightly different especially as he was male - he'd almost 'set' his identity by this point and clearly wanted to take it out on me from the very beginning.
So I think a lot rests on the state of your underlying relationship and whether she values you too. Don't take an initial rejection as a bad sign though. It might be the oppose - my brother was always desperate for my mother's approval alongside total rejection of me (sibling rivalry / attention seeking were very much part of the dynamic in my family hence how a lot of it panned out. Ironically my mothers pandering - which drives me insane when she does it to me - is liable to have gone off the scale and pushed him away eventually anyway!)
'Everything in moderation' is the key phrase I think that strikes me throughout. Don't push, play the long game, a game where you focus on encouraging her to ask questions herself by carefully wording things rather than being more confrontational and trying to plant ideas more directly. That's what chips away at cultlike thinking.
Charging in like a bull in a china shop and quitting job, isn't going to work. You have to respect her decision making (cos she has a right to make terrible decisions) but also stress you care and are worried.