How is Krebs an example of anti-hero in "Soldier's Home"?
Unfortunately I had to edit your question as it actually asked multiple questions. Enotes regulations only allow you to ask one question, so I have focussed on your first question.
When we consider the term "anti-hero" we are thinking of a protagonist of a text who appears in much modern literature. The anti-hero contrasts sharply with the traditional figure of a hero, who responds to challenges and conflicts with courage and self-sacrifice. The anti-hero however surrenders to forces such as inertia, hopelessness and disillusionment. It is clear that when we examine the character of Krebs with this definition in mind, he fits the role of an anti-hero. He is disillusioned, passive and defeated by life. Note the concern of his mother and how he is compared to Charley Simmons, another young man of his age:
"Your father is worried, too," his mother went on. "He thinks you have lost your ambition, that you haven't got a definite aim in life. Charley Simmons, who is just your age, has a good job and is going to be married. The boys are all settling down; they're all determined to get somewhere; you can see that boys like Charley Simmons are on their way to being really a credit to their community."
However it is precisely this that Krebs is unable to do. After his experiences of the horrors of war, he is unable to "settle down" and "get somewhere," because he has decided to not participate in life, finding it easier to have no commitments and to live life on the sidelines. All of which, of course, marks him out as a typical anti-hero that does not conform to society's demands and pressures.