Wow, that is a lot of questions. Here are a few replies:
1. Yes and no. Willy is both stuck in his role as a salesman, and thus unable to re-train himself for a different future (his tragic flaw) as well as at the mercy of society at large, where the role of the salesman is dying rapidly. Thus he has become both obsolete as an individual as well as obsolete in his role at large.
2. See #1, as I said it is a combination of both internal and external failings.
3. Willy's dream is to be rich and successful, like Ben. There was nothing wrong with the dream per se, but rather Willy's attempts to realize the dream were all wrong, because he never took risks.
4. The American Dream is primarily showcased through Ben, who is successful and rich and achieved his wealth at age 21. It is further manifested by Willy, who envies Ben's success and dreams of duplicating it, but does not know how.
5. I would say Biff is the antagonist to Willy - he is the one who pushes the action forward at Willy's expense.
6. Happy becomes the antagonist in this case. Happy is the one who criticizes Biff constantly for failing to reach his full potential.
7. Linda is the stock mother, a character who cares about everyone in the family and merely wants to keep it together regardless of the failings of each of the individuals. It is this personality that allows her to overlook Willy's and Biff's failures, and to constantly support them and encourage them.
8. It functions as a social commentary in that many of the characters are stock characters, i.e. they fail to develop or learn from past experience. This inertia allows Miller to comment on society at large through these easily identifiable characters. If he had made them more complex, the message would be muddied.
9. See above, but primarily Ben represents the person that Willy wants to be.
10. Charley, the successful neighbor, and Bernard, the equally successful son of Charley's, are presented as a contrast to the Loman family. They are the successful versions of Willy and Biff, and allow us to see just how unsuccessful Willy really is. Thus they provide the context for Willy's failure.
