Well, I think the point of the kid getting caught was that key to the storage room. They threw it in the river, after talking about how lonely a river meeting the sea was. Maybe the key was a symbol of all the crap we preoccupy ourselves with in the 'real world'.
Also it was the end of the realationship between the narrator and the kids mom.
I think that K and Sumire were two seperate people, but I like that theory.
My question is, what the hell was with the last paragraph? Bloodstains that seeped inside? WTF was that about?