Boudica that was really helpful to read, as I have no experience with selective mutism. It really helped me to understand a little bit what it must be like for Marks d, and also....How encouraging that you were able to overcome it!
I used that technique to get my youngest to talk more in situations when she usually wouldn't. Basically starting small, practicing with her, rehearsing with barbie dolls, walking thru different situations. She was shy and getting excluded at school from kindy on. She's so strong now, a few years later. She has a good group of friends which helps.
The corner shop was a wonderful place to practice things like asking for something, and counting out change. We have to be a bit more creative these days, esp with covid.
Mark my fried was going thru similar feelings last year. Her d has downs sysndrome, and she was very anxious about her supports disappearing, esp as she has to work and couldn't be there to drive her around. They're both going good now. G has a lot of support in place, learning life skills, and some work experience in place. My friend has gotten used to leaving her at home. She's learning stuff. One day she'll be catching buses on her own, and maybe even having a boyfriend. Just like anyone else. Only different.
I look back now about how I felt when my eldest was approaching school leaving age. I honestly didn't know how she would go. (As it turned out it was taken out of my hands, which was incredibly painful to not be involved in her life at that stage)
We all worry about this stage I think. It's the nature of it. Your d's special considerations might give you more concern, but the rules are the same. The baby chicks have to take their first flight, leave the nest. We can help them, if they let us, but we can't do it for them.
The worst thing is to worry. If you can turn your worries into productive action, like picking up the phone and finding out options for school leavers, or researching things she might be interested with. Maybe talking to the school counsellor or someone that has made a connection with your d? You won't get all the answers, but it may help to alleviate some of your concerns.