how do the relationships of the other characters contrast with that of George and Lennie

There are too many points for this space. I'll take your main question and then you need to put the other ones in questions and ask them separately. As you mentioned loneliness is a major theme in this book. Men generally travel alone until they find work. Once at a ranch, they simply work with each other. Close ties are seldom made and they live a life of emotional isolation. George and lennie are soul mates so to speak. They get frustrated with each other. George often dreams of life without taking care of a man-child. Both men go through a sort of monologue where they envision life alone. This is simply venting, they soon settle back into the simple companionship that they both need. In the end, George and Lennie share a bond that is unique in a story that is filled with loneliness.