1. That is just warning message of php structure. Don’t worry, it does not effect on theme functions.
You can hide it by turn debug off.
define( 'WP_DEBUG', false ); @ini_set( 'display_errors', 0 );
The following code, inserted in your wp-config.php file.
2. When user register new account, you will see the notification in admin dashboard.(my image)
3. The same with partner register, you will see the notification in admin dashboard when partner create service.
4. You can use SMTP system to send email. Our document will show you more details: