I have not worked with iCloud but have been looking through some of the documentation. I am simply trying to figure out if it is possible to send a UUID & an App ID (?) to iCloud and have iCloud tell me if it the pair is legitimate - if the UUID actually belongs to a user of my app.
There's no way to connect to iCloud via anything else than the app itself. You can't even get the user's iCloud email on the app.
After May 1st you won't have access the the UUID of a device. What you're trying to do isn't possible.