A few questions (Spoiler)

Here are my answers:

1. Yes.

2. I think she just collected them for the same reason other kids do: because they're pretty. But I think the throwing scene is definitely meant to mean something. My take is that yes, she's naming the characters. But maybe it's meant to symbolise renewal.

3. No, Sawai wasn't the murderer. I felt he kicked Akihiko off because he believes that it isn't as black and white as "he did wrong, he goes to jail". He just can't give up on Naoki as easily as Akihiko dismissed him. I believe that he sees Naoki as his son now. He's disgusted that the cousin feels nothing over the imprisonment.

4. I took it as an uplifting ending. Kozue is now over the grief of her parents and Sawai is now alive again.

I'll try to add something if I can.

1. Yes.

2. To add to the previous comment, they are the emotionally important characters in her past. If I remember correctly, this is the first time she speaks in the movie (aside from telepathy, which I don't count), which is of course a climactic event. It appears to be a movement from a silent life, oppressed by the traumas she has endured, to a new life where she has a voice again and can verbalize her previous feelings and experiences. In effect, she is giving shape to her feelings (in the form of seashells), and then throwing them out. By doing this she reinforces her control over her life and closes up a part of her past (unlike Akihiko, who is still visibly controlled by his). It is a liberation of sorts.

3. I'll add to elcu's response, but it won't be anything significantly different. Remember that Sawai understood Naoki's urges, as he confided in Akihiko near the end. The difference between Sawai and Akihiko is that Sawai does not rely on societal norms and values like Akihiko, but instead acts and feels on an emotional basis. Just like the film, his experience is not dominated by words and rationalization, but by raw feeling and understanding.

4. To be very short, they found themselves and each other. Slightly longer version: Sawai and the girl found peace of mind and got over a very traumatic series of events. I'm not quite sure what happened with the cough, though.