Hi, Nikki!
Am responding not answering questions nor providing suggestions but to ask if your site is still modifying participant records as it did in the past.
In the past, when a participant in a forever course completed the course and got their certificate, another person in a manager role modified via custom script that participants authentication from manual (or whatever) to one of using an external database. But, in the setup of the external database, that was pointed not to another DB server/external DB but to the same DB server ... and thus same mdl_user table.
Is that still the case?
Also, if I re-call correctly, in the 'forever' courses the participants whose authentication was external DB were moved to a group whose permissions were such where they could see the resources of the course and their certificate, but could not interact with any module like quiz/test/assignments, etc. Those were hidden to this 'forever student' role.
Is that still the case?
Think that would be a factor in anything anyone would have to suggest ... and would also relate to your other posting which is really an extension of this post.
'SoS', Ken