@glparker
Do you have access to the web_logs for Canvas Data 2 from more than 30 days ago? This would potentially have the record of the request that was made to update to the due date for the quiz and then show who made the request. However, if you don't currently have access then I think that data is gone since web_logs are only available for the last 30 days.
Since you didn't see evidence of the teacher doing it based on PageViews, it is unlikely that someone made the change by hacking into their account or it would be reflected in the PageViews.
If you have a small enough set of admins that you suspect could have done and have the appropriate permissions to have potentially done it, you could trying auditing each of their PageViews. However, this may be more effort than it is worth.