An employer has already investigated the facts whenever a nurse is disciplined, and more so when one is fired.  Charts are scrutinized and witnesses are questioned.  It's the results of that investigation which are sent to the CNO.  

If a nurse contests the facts alleged by the employer, the College investigates.  If he or she agrees, no they don't - unless the report is deficient on its face.  The person reporting on behalf of the employer is usually an advanced degree nurse-manager close to the situation, and the CNO relies on that.

There is no reason to suspect - even now - that the medication errors Wettlaufer made and was disciplined for were anything but errors.  If the patients involved in those instances died, we'd know about it. 

What I think you're talking about is investigating every patient, every chart, that a terminated nurse has ever been involved with. That's unrealistitic.  And there are 1300 terminations a year.