D..
I haven't read the book, but I saw the movie. I thought it was a good movie, though of course, the subject is so horrible.
From watching the movie (I don't know how different the endings are from the book - sorry), I think that the reason she didn't tell where her body ended up was because they already had evidence from his diary of what he had done. The girl decided instead to have the experience of kissing the boy she had been sweet on - in the end, that became more important to her. That meant that her spirit would be at rest. Her family was able to be at rest because they finally KNEW who killed her, and though they didn't actually catch him, he would certainly be identified once his body was found after his fatal fall.
If you haven't seen the movie, I would recommend seeing it now that you have finished the book. If what I say doesn't make sense, perhaps the book and the movie are very different.
I know this doesn't answer all your questions, but it has been at least 2 years, I think, since I saw the movie, and this is the part I remember.
Dawn