This is probably repetitive but nevertheless let's take a look at the question of Susan's horn from another perspective.
Why does Susan not take back the horn when offered by Caspian? An easy answer would be that it doesn't work outside of Narnia. There seems to be some wisdom in that given that The White Witch's magic doesn't work in our world either (though it does work in Narnia - begging the question why magic seems to work in other worlds but not ours).
But perhaps the real reason is that Susan realizes she doesn't need it. The horn brought Susan and her siblings to Narnia. They were the help that Aslan sent to the war-torn, beleaguered Narnians at their moment of desperation. If Susan were to blow the horn in our world - wouldn't it just call Susan? Obviously, that would be worthless. But it also suggests that Susan should be able to solve her own problems. She should be the one to bring her own salvation.
Now that's quite a statement - one should always pray to God and plead with Him for salvation. And God gets to choose His method of helping us. I trust everyone knows the joke of the person standing on a rooftop with rising floodwaters and refusing the assistance of a boat and helicopter because he's sure that God will save him. OK, so we need a balance - on the one hand we have to take responsibility for ourselves, on the other we cannot insist that "My power and the strength of my hand" (Deuteronomy 8:17) will bring about salvation.
So, what should Susan do? The horn symbolizes God's help, but God's help is her. Keeping it may cause her to haughty and think that she is the source of salvation. So, she gives it away - enabling her to concentrate of God's salvation, but still with the realization that she can be God's messenger is bringing help to others.
We know what happened to Susan. She is overcome with the vanity of the world and gives up her status in Narnia. But perhaps this is part of her salvation and that of others. Peter, Edmund, Lucy and the rest, never tell their story to anyone. No one is inspired by them, no one seeks to follow their path. One day the horn will blow calling Susan to return to God, to once again be His messenger. She will hear the call, and she will come - but when she does, she will bring crowds with her.
All sorts of people will hear her story of youthful passion in God, the teenage cooling off as she substituted the spiritual for a physical focus, and finally enlightenment that only God is worth the time and effort. They will resonate with HER story in a way they never could with the story of Lucy and Peter.
Once and Queen of Narnia always a Queen of Narnia. Queen Susan of the Horn will always be the source of 'magical' help and salvation for others.