Modifying an WP template

First, inside my WP website I have WP Job Manager Field Editor module for the listing fields on the website. I want to create a new field type - textarea box, and with the use of Custom Field Widget (is part of the module) to be able to display on the front-end this type of code (to be displayed graphically, not the code): <div id="xxxxxxxxx"> </div><script async="true" type="text/javascript" src="[login to view URL]"></script> (the code will be different for every listing, it will be inserted by the user on the frontend-add a listing page and it needs to display some pictures and text from another site, not the code on his page) Second, on 'Add a listing' page, I have one multi-select field. I can choose some selections on this field, I hit preview, and on preview page I have "Edit the listing" or "Post the listing". Problem is if I hit "Edit the listing" it gets me back on "Add a listing" page but my multi-selections are removed on this field only, the other fields are fine. I need the options to remain selected on this field if someone hits edit on the preview page. Also, the listing page is divided in 2, the main content and the sidebar. On mobile version of the listing page, the main content comes first, then the sidebar. I would like to move 2 widget boxes from sidebar before the content, in mobile version of the website, and a third box from sidebar somewhere between the main content boxes, without changing the order in desktop view. At the end of the project I need to know which files were modified.