1. Layouts are synced across devices. Deleting a layout removes it from all devices. I recommend thinking of this in terms of users rather than in terms of devices. If a layout is assigned only to you, then you can delete it without affecting anyone else. If a layout is assigned to you and other users and you don't want to use it but other users do, you can remove yourself from its Users field.
2. Deleting a layout removes it from all users and all devices.
3. From the Repertoire > Layouts list of the website, you can do a few things: search and batch-delete layouts that are only assigned to users who are no longer active in the band; search and batch delete layouts that are sized for devices that are no longer active in the band (i.e., if you get rid of an old iPad with a 1024x768 screen, you can search for that screen size to remove layouts that were created for it); click the Find Duplicates button to identify layouts that have the same name and size and merge them.
4. Yes, by clicking Help > Utilities > Load Standard Layouts.
Keep in mind that when viewing a set list, you will only see layouts that are active, assigned to you, match your screen orientation and either match your screen size or have the Scalable option turned on. You will only see layouts for other users and screen sizes when going to the Repertoire > Layouts list.