Hi OP - do I remember your username from the Oxbridge applicants thread some years ago? Anyway, you sound lovely - as you did back then too.
I haven't read the full thread, just most of your replies, so sorry if this has been said many times. But what screams out to me is that your daughter wants you to feel 'responsibilised' for her situation. It's easier for her to project her unconscious angst / insecurity onto you. You feel paralysed by all this, in the same way she does. But the endless projecting is just her go-to psychological defence mechanism - it means that she doesn't have to challenge and berate herself because she's doing that to you instead.
Fundamentally, I'm sorry, but an autism diagnosis is NOT a trauma engendering PTSD. Autism is a spectrum that many people are on and millions don't even know that they are. She is lucky she has parents who took the time to have her diagnosed. What would she have preferred - that you just ignored her struggles, didn't seek professional help and let her get on with it? If you hadn't had her diagnosed, she would no doubt be using that now as a stick to beat you with.
As I'm sure you're aware, a massive proportion of students at top unis have some degree of autism. Autism is not a barrier to going to uni. Some may say, for certain courses at certain unis, it's almost a prerequisite! Tell her this. There are millions of autistic spectrum people at unis - why wouldn't there be?
Stop feeling blamed. You are a human being and have done your best as a mum. She will understand this one day. This is just a point in time.
Also bear in mind that she is highly likely to be going down a self-aggrandising rabbit-hole online with loads of people telling her what she wants to hear - "speak your truth," "you are not a label," "omg your parents have caused your PTSD," etc etc etc. This is the Gen Z online culture and it's very toxic - way too much navel-gazing to be healthy.
If she continues with blaming you around this diagnosis, ask her would she have preferred to have had parents who didn't care enough to bother - because there are millions of those.
The world does not care if she is autistic. The world cares about how she treats people. I think she needs to get out of her own head and do something for other people. You mentioned she likes animals - that's fantastic. Could she work with cat adoption or some other animal charities? She has so much to offer and needs to reframe her situation as 'ask not what people are doing for you, but what are you doing for other people' (or something along those lines)!
Would she volunteer in a turtle or cat sanctuary abroad for a few weeks (or longer) - have a look at GVI or similar organisations?
But just stop pandering to this 'diagnosis' nonsense. The fact is, she is who she is, diagnosis or not. While she's blaming you, she's avoiding taking responsibility for anything. I wish you all the best.