Depending on the nature of the fan mail, you can get a reply from Kristen Stewart. According to the Kristen Stewart's fan site, there are hundreds and thousands of fans who send her fan mail. She may have no time to go through all the mail and get the time to reply all of them.
Doubtful. Those people have staff who open and answer the thousands of mail and gifts they recieve for safety and time reasons. Requests for pics are stamped with a signature stamp by staff, most gifts are donated and letters/email are answered by staff. Only a few that stand out MIGHT be passed on to the celeb.