Van Cliburn Amateur, I hear, is world class. You practically need to be on the level of the normal Van Cliburn winners to win

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2. There's an amateur Chopin competition in Chicago I believe
3. Don't know
4. A reward, public recognition, and the ability to give concerts that one that one would never give because one chose a different field. I don't know any particulars.
Check out the van cliburn site.