How to select photos which are not tagged or set face(s) in Photos

Unfortunately, you can only side-step the issue by clicking the Albums tab > Faces, and select each found face & delete it. You cannot bulk select, so this must be done manually for each face.

You should log an enhancement request (or three) at the Photos Feedback page.

Kind of. The metadata is there and can be read by Photos on the iOS - I'm just not sure that there's a good way of automatically doing it and grouping them as you want.

But try this: launch Photos on your phone, hit the search magnifying glass, and start typing someone's name. It'll give you a menu list of all of the people, locations, albums, keywords etc that match your search.

My guess is that 'smart searches' will be coming soon, where you can then save that search and then later just pull it up as though it were an album. You can do the same in Photos on OS X but it doesn't sync that album over to iOS ... yet.