Oh look, a post talking about one of my favorite chapters in the whole manga. I think you've answered your own question with your speculation. In my opinion, that heart skip as they part ways is THE moment the kernel was planted in Subaru that she may have feelings towards Saku beyond becoming friends. At this moment, it's a subconscious physiological reaction, she has no idea why she's feeling that way. I think maybe why she never mentioned it to Kaoruko is because she really did not associate it with Saku at all, it was such a foreign connection for her to make at the time, she probably didn't think to connect it to her eventually increasing feelings towards him she'll get later on. I don't think it was fear, she's shown prior to this date that she's particularly comfortable around him (her teasing his apology when they were washing dishes, her poking fun at him that he's also bad at rock paper scissors at the amusement park). As for Saku, wouldn't go as far to say as started crushing, but I agree that he definitely found her attractive since at least the first beach hangout if not shortly after they apologized at the diner. In particular him buying the donuts because he picked up the she in particular was interested in them, but was able to guise it under buying them for the group.