I feel so sorry for the both of you.
It sounds 100% like PDA
Even the wanting the 'babyish' items - being a baby means there are no demands, you are safe, you are looked after, you want to go back to when things were simple and easy.
how was she as a toddler - did she go to nursery at all?
did this all start happening at school.
Was Brownies and swimming after school? they are both quite demand heavy and I can see why they may have become overwhelming.
She seems to be spiraling, as you spiral too, and that makes her fear of rejection and abandonment greater, and she meltdowns more - its feeding into itself. Its no surprise that self sabotage lies heavy on the sholders of those of us who have PDA, and who watch our kids with PDA cut off their noses to spite their faces, from the outside.
It can and it will get better, although I know the knot of fear and desperation, and feeling of being trapped. Of having your hand forced to home school when you know that its impossible. My daughter didnt break until year 7, and she was never physically violent to me, but there had been absolute shockers of meltdowns over seemingly ridiculous reasons before there, but her other medical issues had also help mask.
She is 25 and just finishing uni this year. Living at home, and threatening to quit maybe 4 times so far this year, but I think she will stick it out. Schools havent really been much cop - we did eventually get an EHCP which did help a little, but she didnt follow the same pathway as other kids. But when she is doing the things she wants to do, wowee, she is building an incredible career path on the side of Uni. That tenacity and drive and bloody minded focus will take them far when they find their passion.
But when they are little, and they are exhausted by keeping it together, because they REALLY want to be liked, and they REALLY dont want to lose face in front of their peer group, but their feelings are huge and they live in fight or flight - well it all comes out as soon as they are at home. And its horrible, yes, its really horrible, and you need some help.