It's good that you are thinking about this stuff,
but this thread has been started before.
Check it out,
it might help you along.
As to your question about why the Valar didn't get rid of Melkor right away, try to look up an essay called "osanwe-kenta." You can find some info on this board if you do a search, PM me if you need any help finding the actual text. In short, Melkor could keep his mind totally walled off and project only what would win him Manwe's good favor.