I agree wholeheartedly with Cinderella.
I assume you are now en route but, when you get back, you need to have a long hard look at what is going on here.
You haven't given much info about your DD but what you have given, i.e. form for refusing to do things/go places at the last minute, made me think immediately of my DDs who have AS.
We've had many occasions when they have wanted to do thing, go places, take lessons, join groups, etc but when the time came, anxiety took over and they couldn't do it.
Holidays are similar. My DD2 looks forward to going away but just leaving for a weekend in a familiar place can have her hiding in her bedroom, unable to come out. She just calmly sits there saying she is not going.
We had no idea either had AS until DD1 was 12 and started refusing to attend school. Autism was so far off my radar that it took a 2 hour conversation with a psychologist to make me even consider it. He was right.
Behaviour caused by AS can often make the child appear to be very selfish and spoiled and, if I were to describe the behaviour I have encountered from my DDs on here, there would be lots of similar comments about putting our foot down and not letting the child be in charge.
As for just putting her in the car - well how many people on here have tried that with a 12 year old? We made the enormous mistake once of trying to force DD1 onto the school bus. We quickly realised that it was far too dangerous and vowed never to try it again. I would never suggest that anyone else tried it.
Google Tony Attwood and girls with Asperger's and see whether any of what you see fits your DD. If it does you need to ask for an assessment so that she can get the support and understanding that will enable her to overcome her fear or transitions and unknown situations otherwise she is going to be restricted in what she can achieve as she gets older.