I thought you might be interested in a brief update.
C has managed to keep her clothes on (although her shoes and socks go flying at times- but who can blame her for that?) since we decided it would work a lot better if we had a much (much) shorter session time of about 7 to 10 minutes.
Most importantly, she has been able to peacefully let me know when she is not interested in having a session, and that seems like a step in the right direction to me.
I've said this before, and I'm saying it again: Any time one of my clients feels safe saying "no thanks, not today" to me, I know I'm on the right track.
Now I just have to hope/trust that, at some point, she'll feel safe enough to say "yes" more often.