I have pushed out several bug fixes and an update to the admin area today for this plugin i have also taken a look at the remove unused images functionality and as far as i can tell its not going to be possible without risking removing images of other plugins or some custom db querys.
My original plan was just to search through all posts and users grab any profile pics and cover images along with files from posts and then remove any thing that is in the s3 bucket that is not in that list. But if someone added a plugin that added images somewhere else that would remove them. the other option is to search through the DB for any occurrences of the s3 bucket and get the file id from that but i dont believe that's possible with the current NodeBB functionality.
So the only option left would be to write custom query's for both mongo and redis and interface directly with the db. That's something im not entirely sure plugin should be doing and im not even sure that's possible with redis.
As for the individual path per user i dont think that would add any benefit to the plugin as everything has a unique id.