I don't think that's necessarily true. Not every character has to be a forshadow. She was isolated to Onigashima, really not an issue here.
For all his planning, Oda does make things up along the way so it wouldn't be a first time.
Vivi is a central character of Alabasta and Baroque Works sage, yet when Miss Wednesday was first introduced Oda planned for her to be a basic enemy.
Maybe I shouldn't claim Oda didn't plan Yamato as a fact, but I'm def skeptic.