While the story does have some pacing issues, I personally think it's to be expected of a one-shot. Most of the romance one-shots I've seen capture a small moment in time, and at the very least, you see a sense of development here.
At the start, Torii more or less took it as a given that Kai would simply agree to hang out with her, and got bitter when her offer was refused. When they do become closer, it's because Kai was there for Torii for several difficult months, having put in the effort to develop a relationship.
As for Kai, she admits later on that she'd probably gotten a bit too invested in the piano, and didn't originally understand her teacher's advice to go to a non-music school until after spending a few months without being able to play.
It might have been nice if this had been a short series, maybe a single manga volume worth of chapters, but the one-shot gets its point across fairly well.
As for the accident, the previous two cases of Torii imagining Kai's hands being injured immediately snapped back to reality, clearly showing that they're fantasies. When Kai's hand hits the side mirror, the next scene shows that it really is happening, and Torii realizes she didn't want it to happen.
