You have asked more than one question and according to enotes regulations you are not allowed to ask multiple questions. I have therefore edited your question accordingly.
Key to answering your question is understanding that in the Jewish world, pigs are considered unclean and cannot be eaten. The author of this parable is thus showing just how far the younger son has fallen into degredation and poverty. He has gone from being wealthy and popular to having to work looking after animals that his religion states are unclean. He is so hungry even that he is willing to eat the food of the pigs to sustain himself. We are clearly presented with an image of the younger son as having fallen to the absolute nadir of his existence. Thus allegorically, we can argue that this section of the parable symbolises a human being's separation from his spirit and association with unholy things. The author is keen to present a picture of a sinner who has fallen as low as he can go before being restored into the household of "the Father," who is God.