@NZPIEFACE
Yui didn't have much time to tap into Hiroki's strength since Daiki was already moving to kill Hiroki's body with his knife. Maybe she also thought killing her body would kill Daiki (and leave his original body soulless), but her last words definitely felt like she wanted to keep living. If anything, her top priority was to keep Hiroki's body intact. Chalk it up to a snap judgment from immediate danger.
@Jixbii
Yui (in Hiroki's body) mortally wounded her own body (controlled by their father, Daiki). Either she died right after the Cross Method time was up, or the Cross Method time limit was absolute and returned all swapped souls to their original bodies if possible (and since Yui no longer had a body to return to, the magic itself had to kill her).