1. A strong theme in this book is trying to sustain innocence. Is Holden's desire to protect children from the "dirty" things of the adult world an impossible one?

In my opinon, yes, it is an impossible one. Growing up and facing the truth behind the adult life is a  part of living. he can't "protect" children from the world. it's the circle of life, and he, and all other human beings, will have to face it.