No, he really changed, he hasn't been pretending the whole time.

Converts are often the ones with the greatest faith. (This isn't just true for religion. I have seen people who "convert" to healthy eating do this or those who suddenly become active in a political or social cause.) Edmund was physically (as well as spiritually) saved by Aslan in a way that it would be hard for him to try to explain away or deny. While Eustace's does change his behavior after his "conversion" I think that after Lucy Edmund is the one with the most faith.

I do think that Edmund's faith gets "the big picture" and, thus, he is willing to think outside the box, bend the rules, etc. I haven't read very much of C.S. Lewis' writings beyond Narnia (interviews, his letters, his nonfiction, etc), but after finding the basics of his conversion I've wondered if there is a bit of himself in Edmund. (I also wonder if that factor influenced Susan's fate. It almost feels like he was going after a part of himself and his past he didn't like.)

Lucy is the one most people can't really manage,the pure, shining beacon of Belief.

This is one reason I am surprised at how many people say they identify with Lucy when it is very, very rare to see that type of faith (again not just religious faith) in people.