The way we manage this by allowing users to link multiple identity providers to their login. So if they drop back to iOS 12, they can use another social provider (or reset their password) to sign into their existing account so long as the e-mail matches.
This obviously doesn't work for anonymous sign in, but there's not much we can do about that.
Don’t create multiple Bundle IDs of the same app. If your app has different versions for specific locations, sports teams, universities, etc., consider submitting a single app and provide the variations using in-app purchase.
You can sign and deploy apps to your own device with Xcode. This is all explained immediately after the “no jailbreak required” dot point in the README.
For this, they’re most likely reading the phone number from the physical SIM card left in the device.