In answer to your first question, the poem is obviously about Jesus Christ, and many critics have looked to Simon as the Christ figure in the story. He is kind and helps everyone, doesn't seem to take sides, and in the end dies for the boys sins or because of them in some sense.
The poem also has several other themes that fit in with Simon's character in the book. The ideas of acceptance of one's fate in some ways coincide with the fact that Simon appears to accept his fate and does not fight to change it at the time. He also mimics some of Tennyson's examination of man's subservience to God's will as Simon appears to play a role that he cannot avoid, he has the vision, he finally understands but is killed by the boys in a frenzy, preventing them from learning the truth.