@Lilliwyt—
The issue isn't one of
trust. it would be a violation of
trust for Kase to begin a relationship with someone else without first informing Yamada that their relationship had ended. What Yamada fears is that Kase will stop loving her because Kase will be in continual contact with someone whom Kase will regard as better. And Kase could not, by some
act of will, ensure that she would always love Yamada and never love someone else.
Yui doesn't see herself as charming. Yui isn't charmed by herself in the way that Tomoka is charmed by her; Yui isn't charmed by herself in the way that the audience is charmed by her.
Anyone could
crush Yui in just seconds, by demanding “How could Kase-san love someone like
you?!?” because Yui doesn't have the answer. And Tomoka doesn't recognize that Yui couldn't answer the question, so she doesn't work to help Yui to see the answer.