and yes, as @Trianglesandcircles1 say, the potential of any neurodiversity or personality disorder doesnt make her behaviour acceptable. Understandable perhaps, but it doesnt mean that you have to indulge it.
Especially given that you seem to be the scapegoat in the triangle of narcissism/golden child/scapegoat
Its really hard. You cannot change how she behaves and she reacts. Whether she is willing to try, or whether she is unable to, due to whatever reasons, it is actually quite unlikely. She has found a way that serves her well - be that due to awful parenting / early life experiences or undiagnosed neurodiversity, or a toxic mix of all theree - her ways of getting her needs met are that of a young child, and they seem to have been so extreme that they have endured into adulthood. Im sure that she can be pleasant, funny, charming and attentive. But its when things go wrong or needs arent being met that either the panic or the fury set in, and the histrionics come out. In the past that has set everyone running around trying to solve and pacify. Ultimately her needs are met.
Golden boy probably carries his own baggage, but like your mum - thats not your problem to solve. I assume he probably uses the same techniques as is a bit of a mini me for your mum. But his approval will only come while he is agreement with her, and does what she approves of. they both probably feed each others neediness and adoration.
You are the saver. But also the role of saver means that responsibility is passed over, and as you say it is your fault when it goes wrong, even when she goes against the advice that you gave. And no praise when it doesnt because 'she did it'
You cannot win here. Ultimately you need to learn how to step out of her game.
Back to the start - you cannot change her behaviour. The ONLY thing you can change is how you react to it. At the moment you are still trying to fix it.
You need boundaries.
You need to to discuss with your own family what you can and cannot achieve.
To be honest Id be twempted to take the wrath (and I KNOW that has awful feelings for you, due to her conditioning of you, but honestly the light off your shoulders in the long run would be worth it) and tell her that you cannot help her. Thats she doesnt have the paperwork and you cannot do anything. Direct her elsewhere. and then refuse to discuss it again. I think someopne suggested it upthread. She is far far from destitute. So wont starve. She wont be made homeless. She may run up debts, but sounds as if she is on the ball enough to not do that. But what she is wanting is actually impossible.
I expect there will be another disaster. But you are allowed to step back. You are allowed to not help - to pass her to agencies much better placed and local to her. You are allowed to not be drawn into her dramas