This is awful. Similar to the deportation of Jews in the 1940’s in the Third Reich.

Apples eco system does have pretty good safety features. As far as I have heard, Apple is indeed not up to sell user data outside their eco system. However, they collect data from their users and use it for their own purposes. It is always a good advice to go through each setting (of phone as well as laptop) and evaluate if it is needed for functionality (e.g. location services or access for the camera or contacts). Not only will this measurement reduce the amount of (usually) unneccessary data that is being transmitted otherwise, but it will also improve battery life as well.

Do you mean iCloud Mail? I assume it is as secure as all the other iCloud services themselves. Personally, I don’t use iCloud (except for the “Find my” feature). But you can set up any third party Email service with the Apple’s Mail app. I use Mailbox.org as email provider and have it set up with the Mail app on my iPhone. It works flawlessly. Third party email providers often come with additional costs (the lowest tier with Mailbox.org was 12 € each year when I switched from a freemail provider a couple of years ago). I don’t know if iCloud Mail is free of charge, other than additional storage space.

I think with a sufficient wall-thickness your box would be waterproof. If I were you I would test it first by printing a small box like you described you would, and put a piece of paper with ink writing inside. Then submerge it into water for a few hours. After that, let it dry thoroughly and saw it open to see if the paper inside has gotten wet or the ink writing has been smeared/diluted.