I would imagine that this is similar to the same shared security model that you would use within AWS and other Cloud Providers. At some point you have to point

I would imagine that this is similar to the same shared security model that you would use within AWS and other Cloud Providers. At some point you have to point to their documentation and how they do things as being out of your control but possibly audited on its own. But if you are really worried for iOS it is optional. I don't see any capabilities regarding your first question outside of testing specific devices through TestFlight, but I would think it is fairly safe to assume that Apple has put a lot of work into making sure this works consistently between devices. It is also entirely possible that, like working with Cloud Providers, that there is a difference between what normal users can do and what can be done with NDA's and other contracts signed.