Retaking year 11 is a thing (which is opposite to what Octavia64 said), a friends child is retaking year 11 this year. They do have an EHCP so don’t know if that makes the difference? They missed most of last year, sat 2 GCSEs last year and doing 5 this year.
I can’t really help regarding additional access requirements I would say push school for more support. While there are early deadlines for apply for access some things can be done later. For example my daughter was switched to sitting exams at home 2 weeks before the exams started.
There are alternative routes to sitting GCSEs in year 11 with everyone else. You can redo the GCSEs, or do a foundation course before A levels or equivalent, or go straight to A levels or equivalent if she gets the GCSE grades. Generally you just need 5 passes, and maybe higher grades in specific subjects.
Missed GCSE content may or may not be an issue depending on her subject choices?
My daughter missed almost all of year 11 due to her mental health and unable to study at home during that period. We dropped the subjects that weren’t relevant to her to ease the pressure (English Literature, Geography & Spanish), focused on the ones she needed to do the subjects she wanted to do next. Her results were significantly lower than the previous mocks but it doesn’t matter, it was enough to get to college and GCSEs are just a stepping stone to the next thing. Maths & English language can be resat alongside a-levels.
She’s now doing well at college, first year studying A-level Maths & Computer Science. Any missed content does not appear to be causing any issues (especially for CS as not all students would have done it for GCSE).
Good luck and I hope you work out what’s going on and how to help. Facebook group ‘not fine in school’ would give some good tips to alternative routes if GCSEs don’t work out.