Easter egg: In The Chronicles of Narnia: The Lion, The Witch and The Wardrobe Special Two-Disc Collector's Edition DVD, in the second disk, choose the section "Creating Narnia." That menu will have three features; select the one entitled "Evolution of an Epic." When scrolling up an down, a flower will come up on the picture of the director's chair. Click that to see a clip of Skandar Keynes' experiences with eating much Turkish Delight.
Easter egg: On the main page press up, and the little lion picture will become highlighted. Press enter to watch a short complation of shots panning around the Narnian landscapes, while the harmonious instumental music plays.
Answer: Spoiler alert: this gives some important plot twists away. Sometimes a bit of unresolved mystery improves a story, and I think this is the case here. But the book partly answers your questions. At the end of the last chapter it is shown that Mrs MacReady thinks the wardrobe is just a piece of furniture. She knows nothing about Narnia. But Professor Kirke amazes Peter, Edmund, Susan and Lucy by expressing familiarity with Narnia and explaining that a wardrobe might well be a portal into Narnia. If C S Lewis had not written any more books after completing "The Lion The Witch And The Wardrobe" Professor Kirke's knowledge of Narnia would probably have been an unresolved mystery. But C S Lewis later wrote "The Magician's Nephew" which tells how Professor Kirke visited Narnia as a boy. The final chapter of this book says he took an apple back with him, which he planted in his garden. It grew into a tree, was cut down and made into the wardrobe. So Professor Kirke was not consciously aware of what the wardrobe could do, but with hindsight, he realised that he had set up a chain of events that caused the children to discover Narnia.