First you point with saying Hiyori want to reveal herself after the war is wrong, she just not reveal herself until the battle start, with make you point meanigless:
Kinemon ask who could threated their wounds and then Kawamatsu coming up with his statementt that he see someone, but he speaking like he seeing a ghost, if it was Hiyori or Onimaru he would say the other that he see them.
Hiyori and Onimaru beeing in Onigashima wouldn´t be impossible, but Kawa speaking her like from a person with can´t be real:
Hiyori=Onimaru= Kawamatsu would say he see one of them.
Someone who is probably death: Kawamatsu word that there´s just no way it was real.
Speaking like he see a ghost...
If it was Hiyori, Kawamatsu would say what he see and not say it would be impossible.