I adjusted your question so that I could address all the parts: we can only answer one question per posting.
A motif is like a recurring theme, and we see this continually throughout Miller's play. Willy Lohman is obsessed with the idea that being attractive and/or well-liked is the secret to success. This would include the theme of "don't judge a book by its cover," which is an idea lost on Willy.
In terms of attractiveness, Willy believes that his son can do anything because of how attractive he is in terms of his physical appearance and his charismatic personality.
Willy has also believed for the many years he has been in sales, that being well-liked gets one through doors that are closed to others. He sees his past as a time when this kind of relationship with customers not only served to make him successful, but gave him a better chance of acceptance with his old boss and in making new sales connections.
We get the sense that his brother Ben was attractive, well-liked, and particularly successful financially.
The problem with this belief is that it is based on appearance only and not on substance. As Willy ages, he is no longer a young and dynamic salesman; the old "methods" no longer work. He has to perform more calls and drive farther to make any progress. Being someone's buddy is meaningless in the new sales arena—at least for him, so things don't come as easily on the job: this means he is not bringing in the profits he needs to. The "good buddy" deals only work for the younger salesmen. Consequently, Willy's boss can't "carry" Willy if he can't work outlying areas, and Willy feels too old to handle being on the road. However, business is business, and making money is what the company is there to do, so Willy loses his job.
Willy's son, Biff, is someone Willy expects can do anything based on how attractive and well-liked he is. This perception only exists within Willy's mind; in fact, a great deal of how Willy sees the world is based upon perceptions that are many years old and no longer valid. Willy is out of touch with the present-day world; this includes the reality of his son's potential for success based on the old "gimmicks" of favorable appearance and charisma. Whatever Biff may have had going for him in high school, the truth is that he lost his scholarship, never graduated high school, and was unable to go to college because he failed senior math.
Willy does not see these things, and Biff spends a good deal of the play trying or pretending to try to be what his father wants, but he is unable to do so based on the reality of his life NOW.
The major conflicts circle around the attractiveness/well-liked motifs. Willy cannot exist in the present day world and holds onto things the way they were in the past. Biff cannot live in the past, and finally has to remind his father of the truth of Biff's life and limited options for success. Willy does not handle these truths well, vacillates between the past and present, and ultimately takes his own life.