1. Wrong. It would be extremely easy to make this completely impossible... by simply introducing other data (signature, key, specific user data, etc.) when calculating the checksum of the file(data). Even if the files are the same, by adding extraneous data the hash becomes unique to that users copy of the file.
2. Of course you would. And I'd bet you'd be wrong.
3. Yes, it is on purpose and Apple even states why... they are standards based data sets that might need to be accessed by or shared with 3rd party services and software. (For instance accessing to your iCloud email account in Outlook on a PC.)
4. Correct. It would be extremely easy to completely encrypt everything... and by doing so you make any features or services that require that minimum of information completely useless to the end user. There's an extremely fine line Apple (any service provider) has to walk to ensure privacy and security and also provide some convenience in either organization and/or automation.