So, I thought I'd give an update on the situation.
She has finally, FINALLY, got a job. It's a zero-hours waitressing contract, so not a longterm solution, but it's some money, something on her CV, and something to get her out of the house. She's been doing it for about 3 weeks, and is doing around 4 shifts a week. Phew.
Meanwhile, her conservation volunteering suggested she lead some sessions. This would have been great experience, but I am not sure it will happen at the moment as she will find it difficult to commit to weekends.
We discussed other options that people have suggested. She had two interviews at Skills Development Scotland, which from what she told me were not terribly helpful. I don't think she told them about her ADD (which is on her), but they seemed to be basically, "What do you enjoy? What are you good at?" which are more appropriate for 14-year-olds than for someone in her situation. She is also poor at self-analysis, so finds it difficult to answer such questions, and there is really no point in saying, "I love cats," if there are no jobs working with cats. I was hoping they would give stronger guidance, along the lines of, "There is a real shortage of people in X,Y,Z areas, do any of those interest you?"
We discussed some of the career options that people of suggested, and she has been working her way through them. We wanted her to go to the local uni open day for radiography and related careers a couple of weeks ago, but she didn't (wasn't sure what she wanted to do apparently, but of course that's partly what open days are for). Frustrating, but you can't make adults do things.
Now that she has the waitressing job, she is taking a bit of a break from job hunting/career planning, which is fair enough I think as she has been at it so long. I hope that after a few weeks or months she will get back to long-term planning - and if she doesn't she will be getting a shove from me.