Hi,

1) You can set the document owner - but here you can set only one user. Second option is to add a special field to the document types where you will list the users - some kind of user selector.

2) You can use WHERE condition in the web parts, or you can use repeater with custom query where you will build custom SQL query with WHERE condition which will meet your needs.

3) You will need check each document and its permissions in a cycle, using our API.


Best Regards,
Juraj Ondrus