Hi Boo,
I apologise in advance that this is going to be be long, but having been through this extensively with my DS, I feel I could write a book on it! Feel free to ignore anything which does not feel right for you.
I'd say it is all about priorities at this point. GCSEs are ridiculously voluminous anyway and I can entirely understand children just opting out of them, let alone those with additional needs.
I think you are right to drop some subjects and I'd say be ruthless about dropping any subject your child is never going to pass and just keep reiterating that if she doesn't get a pass in the ones which are left she'll have to sit them again (true for maths and English anyway). My DS dropped all three sciences and his MFL to focus on his best subjects.
The other thing is keep reminding her she doesn't need to know everything, she just needs to know enough.
My DS, now 18, has ASD and ADHD. He hates writing, is not really academic and struggles to apply himself - no revision whatsoever took place in our house unless actively managed by me.
The view I took was that I wanted him to get 5 GCSEs at C grade eventually because that is the minimum for keeping options open for later on, but I was not stressed about how long it took. Parents are sold this idea that you only get one shot at GCSEs. That really isn't true, you might have to push and nag or sit at a different location, but it is perfectly possible to sit them more than once.
It basically took my DS 3 goes (using resits in November too) to collect his five. Maths was the hardest one to get and it took him 3 tries to get it, but in the end he did it.
The most useful thing for him was me and his teachers sitting down and going through the marking system i.e. if you put that you can only ever get one mark, if you put this then you'll get five marks. Get 50% and you've got a C. Sounds ridiculous but he learned it like a menu and just wrote his exams to it because he could see the point. So much learning feels ridiculously abstract to aspies.
The most difficult part is that it sounds as if your school are not delivering teaching to your DD at this point. I can understand that she is panicking because she feels so behind. I agree with trying to get your LA to provide tutoring if possible, If that is not possible then I'm afraid it will probably mean you have to work with her in the evenings and weekends, but at least it is only a few weeks.
I think it is critical that she has success in at least one GCSE this time round and it is perfectly possible to plan for that. Remember the old joke "how do you eat an elephant? one bite at a time". That'a how you learn to pass an exam.
My suggest would be to pick your DD's best subject, get the course books for her exam board and past papers and Cliff Notes and bash though them with her. Look at other (ideally easy and memorable) resources too - I still maintain that my DS passed his history paper on WW1 based on a combination of the film Gallipoli and Blackadder! There are lots of helpful online resources too.
Most subjects have a limited range of exam questions and it is possible to develop pro forma answers to them. You might not be able to cover the whole syllabus at this stage but it is very possible to conclude after studying the past papers and syllabus and talking to your child's teacher that, for example, there is always a question about characters in Pride and Prejudice and learn a set of numbered points about that. Teach your DD that passing exams is not about knowing everything, but about knowing how to answer enough banker questions to pass.
Once you have identified your banker questions and answers to them, pick one question and answer, break the answer down into a list of points and learn this list of points in different ways - make your DD write it out, type it, write it on a chalkboard, read it out and record it. Whatever you can get her to do. If she doesn't get it, break it into smaller sections and use read, cover, write again and again until she does.
Do not expect your DD to be able to structure answers without your input, she can't, that is why she is floundering now. She just sees a scary mass of information. Your role is to make it bite size.
You want her to get to the point where you say e.g. "Explain how Elizabeth Bennett's view of Mr Darcy changes" and your DD says "there are 12 points, the first one is etc etc"
Learn ONE set answer to one banker question at a time. Don't add another until she is 95% perfect on the first answer. Remember to keep going back over previously learned answers periodically. Praise her a LOT when she sticks to task. IME aspies tend to be younger emotionally than they are chronologically so they need the level of encouragement you would give to a child several years younger and really spell it out. "That was great, you knew all the first six points and the last three, points 7 and 8 were still a bit wobbly but now you know to think especially hard about them next time so I'm sure you'll get them all next time. Just think, yesterday, you didn't know any of these and now look at you. You have worked very hard and you should feel very proud of yourself." Sounds over the top but it works. You are giving feedback and praise at the same time.
Aim to teach in 15 minute bursts with a break in between and a treat of some sort every half hour. 3 x 15 minutes in one evening is plenty. 2 x 15 minutes is probably more realistic.
Lots of schools are very poor at teaching the techniques to pass exams and students are left just learning long reams of information. Actually I'd say success in exams is at least 70% technique.
Now is probably not the best time, but once GCSEs are over please do pursue a statement for your DD. I found it gave access to all sorts of additional support (like speech and language therapy to improve manual dexterity and pattern recognition) and it definitely makes it easier to take up cudgels on behalf of your child when you can move the dialogue from "your child is naughty" to "your school is failing to meet my child's needs".
FWIW gaining that final maths GCSE was a real turning point for my DS confidence. He has always been interested in art and ended up going to college to do a level 3 BTEC in an arty subject. He's a couple of years older than the others in his class but none of them know or care about that. Better still (and slightly to my amazement) he has managed to stick to his course and does (just enough) work to get by.
Interestingly his two best aspie mates have also ended up repeating a year or two to boost their results, a result decided on entirely independently by their mums.
I'll shut up now!