Please clarify. Does user see existing images at Media Library after you added ure_attachments_show_full_list filter returning true?
So what is a problem?
1) User can not insert existing media into a post?
2) User can not upload a new image into Media Library?
If the 2nd, let me know is the checkbox for “Force custom post types to use their own capabilities” option turned ON or OFF. I mean if it turned ON, then role or user should have capabilities from the ‘Attachments’ group, like ‘edit_attachments’, etc.