I suddenly through of this
They dropped the flowers in the performance of bookmark didn't they?
and flowers have been left everywhere in the prologue film
Bookmark is a story about a girl staying at the same place, waiting for the boy to come back
If we connect the two stories (bookmark and the prologue film) together, it can be a girl staying at the garden, waiting for the boy to come back, and in the end the girl dropped the flower (maybe the girl left it as a sign so when the boy came back, he would know the girl was still here in the garden)
And the doll song is all about waiting
The girl used to dance happily, implying she had a happy relationship
Then the boy left, the girl waited for him to come back, and when she saw herself dancing on tv, she thought of the past and decided to keep on waiting...