You can simply delete the Address section on the confirm page and on the Blank_2.php page without causing any problems.
You can turn off the email receipts from your Authorize.net control panel, or by adding a hidden field to the confirm page named "x_email_customer" and setting it to "FALSE".
Adding the custom field should be easy enough as well. Add the notes field to the Checkout form, then update the confirm page to store the field you have just added as a Session variable. At that point you can add the session variable to the email body.