On Ebihara's route, everyone but the protagonist knows that severe weather is expected right around the time of her business trip with Ebihara. Presumably, Hana was aware of it as well, but still sent the protagonist and Ebihara out into a rural area on business. Given her attitude towards Ebihara's all-work-and-no-play lifestyle, her encouragement of the protagonist's partnership with him, and her general Trickster Mentor tendencies, it's not beyond possibility that she was hoping they'd get rained in and have to make it an overnight stay.
Fridge Horror:
In Koichi's sequel, even after the whole truth has come out, Koichi never really explains why he doesn't deny having an affair with Miss Ito when the protagonist asks him about it directly. With the revelation that Ito drugged him to get the picture that she uses as "evidence" of their affair, it's entirely possible that Koichi didn't defend himself because in all likelihood there's a space of time he can't account for, after which he woke up with his clothes in some kind of disarray - in other words, he can't say he didn't sleep with another woman because he can't be entirely sure he didn't. Not to mention the fact that even if he had slept with Miss Ito, it wouldn't be a case of him cheating on the protagonist: It would be a case of him being a victim of rape.
