If the user doesn't opt in via ATT, is it still possible to show an install came from 'paid advertising' without any additional detail?

Unfortunately, this will not be possible. Until/unless your users opt in, events will appear as fully organic in every way.

The reason for this is that in order to indicate 'paid attribution' (even if all user-level details are redacted), it's necessary to perform the sort of tracking that is prohibited under Apple's policy.