Why was Marie Antoinette the only one who wanted to clap at the opera? (and why the second time we see her clapping, no one else would?)
Applause was not the custom at court performances. However, her popularity led others to clap along with her to please her; the silence indicated the increase of general disapproval. Another reason might be that in Marie's country (Austria), music was not as strictly regulated, much the same as court life was not. Her brother, Emperor Josef, was Mozart's patron. Though he probably did not attend Mozart's "vaudeville" performances for the common people, where clapping was encouraged, no doubt he and others at court knew about them. The German opera "The Magic Flute" was first performed at the people's theater, not at court. Marie might have wanted to show appreciation as ordinary people did in Austria.
Why did Sofia Coppola choose to base the film specifically on Antonia Fraser's book?
Fraser's book is one of the few largely sympathetic portrayals of Marie Antoinette, depicting her as a naive but basically good individual who allowed excess to go to her head, as opposed to the callous, detached hedonist she is usually portrayed as. Coppola wanted to depict Antoinette as a person first and a historical figure second.