Also, when adding fields that use iframe (such as single image) to the frontend or when clicking on the add media button of a frontend form, the iframe breaks the page. The content of the page disappears and the iframe content shows up under the footer of the page.
This is the add image button I'm talking about. To allow authors to add a featured image to a post.
clicking on it opens the iframe in the wrong place and breaks the page. Changing the sanitize_callback has no effect on it whatsoever.
Re-order post fields mean using the custom fields with the same IDs as the default WordPress field IDs. The code would be added in the file functions.php when registering the meta box and custom fields.
Regarding the featured image issue. Can you please deactivate all plugins except Meta Box, MB extensions and switch to the standard theme of WordPress then re-check the issue?