The quote refers to Sal because from the very beginning of her story, she expects Phoebe to understand her sufferings. Sal uses the similar experiences she has with Phoebe to express her own sadness. In a way her memories are a confession that Sal struggles to forgive her mother. The way she focuses on solving the mysteries of her mother's life shows Sal could not address Phoebe's experiences.
Independant clause .............................................................
PRO gress,,PRE conceived, PRE cede, POST mortem(?), POST script, PRE amble ... perhaps ?