1- Lord of the Flies is not a fable. Only on the basis that it is more of a Parable than a fable. It is even more of an Allegory than a fable, but moving onto why...
Fable: is defined by its brevity (how short it is) and by it's poetic nature.
-Often the characters will be animals or oibjects that possess human characteristics.
example: The Tortise and the Hare
Parable: A saying or narration in which something is expressed in the terms of something else.
Oxford Dictionary adds that a parable is usually dark in nature.
-Rather than using abstract discussion, a parable always teaches by comparison with real or literal occurences.
2- well I'm not quite sure on this one, though I'll tell you right now Piggy is the literal symbol for Civilization. The Conch is law and Order with a definate connection there by "whoever holds the conch gets to talk" kind of thing. Represents order and some form of government they set up.
Coming from England the boys know what kind of society they grew up in, but really without any adults around the enforce it, the boys fall from the high standard of living to becoming savage and wild.
3- well the boys obviously think they are constantly being watched by the Beast. By the great unknown. They fear it and any way they can they try to appease it to perhaps make it less scary, but in doing this, make it all the more real for themselves. They feel like they are isolated and away from teh rest of the world. Away from the rest of the world of reason. Away from anyone they'd be comforted by in saying that there's no such thing as the Beast.