I'm so sorry op, I havent read through the thread..
But I was your daughter.
I had a rocking horse as a toddler.
I would rock for ages.
As I got older 4, 5, 6 etc it moved to the sofa.
I used to rock so vigorously my hair would matt hitting the back of the sofa.
I would go into my own world, my thoughts were my own, it was an brilliant, great place. I would go through things in my head, educate myself, think, get through things.
I did this until my early 20s. When unacceptable in front of parents. Prehaps 10+, I moved to doing it privately in my bedroom on my bed.
It was an outlet. I had some of my most brilliant moments, rocking back and forth.
I was also generally very fridigety in general.
I was very slim and very healthy due to my constant movement.
In my early 20s, after some boyfriend(s) finding out about it, I eventually grew out of it.
I did it almost daily. I cant explain why except it felt good, I would trance, it got me in my head and focused.
I dont do it now. I dont have any compulsion to do it now, im in my 30s.
My parents accepted it and never tried to stop it. It was extreme but something that didn't hurt me or anyone else.
Keep your eye on your daughter behaviors otherwise. It might just be that she is, me. It was who I was and that was it.
I am a very normal and boring 30 something now.