UPDATED
I have updated your question and based on that providing this answer.
1. I see bunch of API's for cPanel for managing the server. Though I don't find a direct API for creating a new email account, but found a script which would help you creating email account.
Please see this link:
http://www.zubrag.com/scripts/cpanel-create-email-account.php
http://forums.cpanel.net/f42/xmlapi-php-class-111897.html
