OP: "I just don't know how to support her, what we should be doing/looking at... are her aspirations realistic even."
"Does anybody else have a child whose educational direction is so different to theirs? How is it working out?"
To add to the positive replies to your questions: yes and very well: Parents (us) science educated and working in that area, DCs pursued arts degrees. Both working, although one switched from arts to sciences, and the other to an entirely different arts sector, while both their partners have arts degrees and are extremely gainfully employed; a non-statistical sample indicating that just about anything is possible and an arts degree opens doors.
With regard to the 'keep music and art as a hobby' line, yes you can but you will not reach the same level and that may continue to hurt. A music degree is not a road to unemployment - all sorts of careers are open even, yes, postgrad medicine. And one DC works in a field where a lot of people have a practical art background of some kind. Studying art or music takes dedication and organization (see comments re amount of work required for art GCSE for example) and employers know that.
And as for not LETTING a DC do an arts degree - what?! Where would my DC be if I'd followed that path? Not talking to me for a start. OP, your heart is in the right place, well done. As usual, I love some of the comments on here:
Nooka: "We have several people in the family that took academic paths to make their parents happy, and it didn't work out very well." Spot on, engraved on my heart. Also: MidnightScribbler: Support her in pursuing her passions."
In answer to your worry about not knowing how to support her, I agree with posters saying don't worry. Arguably you'd be no better off if she were following the same path you did, as you won't fall into the trap of thinking you know best. Encourage her to get work and volunteering experience. That will help her get her ideas in order and also seems to matter much more to employers than the degree subject.