Rudeus being close enough to hear Bananahoshi's meltdown literally saved her - I'm fairly certain that she'd use the dagger, she might be psychologically strong but she is no superwoman. And she's smart enough for her own good, I don't think that she'd be able to fool herself with "the grapes were sour anyway".
It's kind of funny how Zanoba, of all people, had the answer for how to solve the circuit. Because of his hobby, instigated by Rudeus on a whim.