Thank you for your quick answers!
For the first point, the deletion of the concerning page was successful, but was followed by the next similar error with another media page. I will keep trying.
For the second point, the client doesn't need the link in CKEditor, but in Redirects module to set a shorter url for certain pdf documents. So I cant't explain him, how to obtain the original url/path from Media Manager to set as "redirect to".