1. Should _face files be visible when using the Events or Photos views? I used the Faces functionality briefly years ago in an older version but I do not recall the _face files being added to each event. I suspect the ones I can see are unnecessary duplicates, and safe to delete...
Faces should only be visible from the "Faces" cork board and not as photos in your events or albums. You have probably imported an iPhoto library at some point, when you restored your library from the backup. When you are seeing photos in events in iPhoto, that you do not want in your library, you can safely delete them in iPhoto.
2. Can I delete the _face files from within iPhoto (i.e. is it safe)? I would do so by finding "_face" and sending them all to iPhoto trash. I have seen other discussions saying deleting them would destroy the library structure, but I think that referred to deleting them from Finder.
That is a good plan, if you do not have an older backup, that is not messy.
Don't trash too many photos at once, however. Only a few hundred at one time. iPhoto used to have a problem with the Trash and could not empty it, if it was too full. I am not sure, if this problem really has been fixed. By trashing in smaller instalments, you will be able to check the trash before emptying it.