As I've mentioned on another recent thread in the Parent Room, it would be a good idea to familiarise yourselves about the various cut-offs with respect to student finance (also also accommodation contracts) so that any decisions are no more expensive than they have to be. Ditto course change issues (often very easy initially, left later may involve extra work and perhaps social issues that may be harder for her)
Has she applied for DSA and got supports in place? What has her gap year been like for her and what help is she getting with her mental health issues? I think that's really important.
Sometimes all the stuff around freshers can increase the anxiety levels for those who have social difficulties and I am sure there will be plenty of threads across The Student Room to show her that there will be many others in similar positions - and even those who do not have 'anxiety' as such nor OCD will have anxieties about how everything is going to go. (You'll have to use your knowledge of her and what social contacts she currently has to know if that's going to be a good idea for her)
It may be that she just has to feel what she feels (and she absolutely does not need to know what you are feeling!). If she's really shut down about it all and doesn't take the steps towards going (eg accepting accommodation offer), she just won't be going... which is a different set of issues but it's an option. Do you have any sense that she really has picked the wrong course? (She wouldn't be unusual in switching courses even if she does start with history - she may need to be reminded of that)
You could quietly do the work now without reference to her so that you are ready with information if she opens up at all. Some years back I had an extremely precise Clearing plan, with all the phone numbers and order in which to use them... whose existence I did not divulge to my elder son. We didn't need it, he was accepted for his first choice... but there were nasty surprises in the results which upset him and he didn't cheer up until he had accepted his room offer. Which we had to make him do. And he does not have mental health issues and was definitely not on the wrong course.
There are times to encourage independence and let our children take responsibility as adults, and there are times where we need to remember that robust, directive support may be no more inappropriate for our particular child's disabilities in a particular instance than something like a walking stick or a hearing aid would be for some other disability.
(I have another child who wants to go for 2024. He'll be 25, he has more MH and neurodivergent diagnoses and diagnoses in the pipeline than you can shake a stick at and a lot of days I think it is the most bonkers idea on the planet... but we plod on as if it wasn't and hope for miracles. Certainly when he was put in front of a tutor it was as if he was in the presence of someone who spoke the same language as him for the first time in his life)
Good luck! Long breaths out!