on this page:
http://www.tnpi.net/wiki/Install_mail_servers
there is no 'x' inside the mysql option for the port build, although it's highlighted.
you need the 'x' for dovecot to function if you're using mysql to authenticate.
No, you don't.
At least not by default, using the toaster as intended. Dovecot will use the vchkpw auth method, and auth_vchkpw may be compiled against the MySQL libs, but doesn't have to be (it'll work equally well with the cdb backends). Neither option requires the dovecot auth_mysql module.