The original question had to be edited down. I invite you to resubmit the other questions in separate form as they are intriguing questions. The atmosphere created in the first two paragraphs of the novel is a deliberate one. Steinbeck understands very clearly what it is he wishes to do and the atmospheric conditions he wishes to develop with the first two paragraphs. They take a broader element and narrow it to the main characters, men we will be following back into that broader thematic and literary element.
The natural condition described in the first two paragraphs helps to establish both how the novel opens and how it will close. The naturalist detail helps to bring out the world in wihch Lennie and George live. The atmosphere of the "golden foothill slopes" that "curve up" and the Gabilan Mountains is the backdrop to all life. This includes the lizard that "makes a great skittering" among the leaves when he "runs among them." At the same time, the "rabbits" and raccoons help to establish a natural world into which Lennie and George will settle. The path that moves "through the willows" helps to usher them into this setting. In this, the atmosphere is one in which the natural world serves as the backdrop to the story of both men. In this light, the atmosphere created is a deliberate one to serve as a backdrop for what will transpire.