How can I ensure someone will always receive the email notifications sent?

If the employee has unchecked 'Get Notification Emails' in their My details > My Details (dropdown menu) > Work (also found in People > Employee name > Person (dropdown menu) > Work) but not selected another employee to receive instead of themselves then a fallback can be setup.

Go to Admin > Settings > Email > Email and add an alternative manager to receive the emails for leave and sickness.  You can specify a specific email address or a marker for a client admin.

You can also specify a Carbon Copy (CC) for every email sent to also be sent to another email address.