4 ideas
The café's central rule is that nothing done in the past can change the present: the dead stay dead, and the departed still leave. Because the outcome is fixed, a trip back is useless for fixing events and only useful for changing what the traveler understands and feels. Regret is treated as a problem of meaning, not of facts.
Fumiko goes back to the café meeting where her boyfriend Goro told her he was leaving for his dream job in America, a conversation she had cut short out of pride. This time she listens, and learns that he felt unworthy of her, and that his parting wish was for her to wait for him. The breakup still happens in the present, but she returns having heard what he could not say before.
Kohtake, a nurse whose husband Fusagi has early-onset Alzheimer's and no longer recognizes her as his wife, has already accepted his memory loss. She goes back to collect a letter he had meant to give her, in which he admits he knew he was forgetting and asks her to be his wife rather than his nurse. His illness is unchanged, but the letter changes how she means to be with him, as a wife instead of merely a nurse.
A visit to the past lasts only until the coffee cools, and staying longer means becoming a ghost stuck in the chair. This deadline adds urgency to the encounters and presses travelers toward what they came to say or hear.