I had to edit your question as it is too demanding to be one question, thus it will be answered a little bit more generally. Split it into two if you want to ask two questions.
Mood is the feeling the audience receives based on what they read.
In chapter one, when the boys first land, or rather crash, we feel a mixed mood because characters have different perspectives. First, Ralph feels free and almost celebratory, but Piggy's definitive rationality flares up and concern mounts almost immediately. Who wouldn't want to be on a deserted tropical island right? Sounds fun! Wrong.
As the story moves along and the boys move from civilized to savage, the mood feels competitive, somber, depressing and ineffective. It feels like rescue is impossible and finally by chapters 11-12, hopeless.
As far as symbols go, chapters 9 & 10 go into great detail about the Lord of the Flies itself an allusion to Satan which we know is evil manifest. Simon then becomes the symbol of the Christ figure. Simon tries to be kind and giving to all and most of all, at his weakest moment, a Savior to the rest of the boys from the idea of the Beast. No boys will hear his message and his life pays the price.