Normally an employee number is a fine PK. It is simple, unique, and unchanging. Unless you are my company that gets sold multiple times and you end up with a new number each time. Or you are in the admin office trying to log a new employee and waiting a couple of days for corporate to assign a new number. That is why an autonumber is always a good choice. In this case you can add a new employee without an employee number. However you can force it to be unique, but not required. That way you can add someone without an employee number, but it will not allow you to assign duplicates.
Normally you would not delete records. Lets assume you only count training that has taken in the last year. You can now show that and calculate the training by querying the records with those criteria. You would not go back and delete the training because it may have historical value. Even if different course had different expiration you could handle that. In the course table you would add a valid duration field (12, 24, 36 .... months). You would then be able to show current training based on varying duration.