CKEditor can not parse JSON response
The JSON response is used only when you paste an image in the content, for file uploads from the dialogs you must use the normal javascript response
What they have in their example in the second editor works exactly the same as you put in your UPDATE.
In response they have Content-Type: text/html and content is
<script type="text/javascript"> window.parent.CKEDITOR.tools.callFunction("92", "\/userfiles\/images\/side-nav.jpg", "");</script>So, there's unlikely to be another solution.