After a staff departure in my department, I filled an open management position on an "interim" basis for over two years before transitioning to a more permanent status in the same position. Should I represent this as two separate CV entries? Should I present the entire period as a non-interim position?
If it were me, I'd keep it simple and just list as non-interim. It's more about what you did than what they called you. Your responsibilities remained consistent (presumably) so go with the regular title.
I would separate them by actual formal title. This then matches up with anything your background check might find.
In the job where you were an interim, if you retained your former title, I would make sure to put selected to be Interim Manager of the department from Aug 2014-Aug 2016 and include your duties.
Then you can put in the promotion in a separate entry.
Both are equally valid. Personally, I prefer to show that it was a separate employment, I withhold a few responsibilities from the temp position and put them into the permanent one to make it look even better.