How can I add Mailing Date to the Mail Details Report?
I used a combination of these four resources to learn how to customize reports to add columns and filters etc with some tenacity and very minimal php and mysql background:

https://wiki.civicrm.org/confluence/display/CRMDOC/CiviReport+Reference

https://alastaira.wordpress.com/2011/03/17/civicrm-creating-a-custom-report/

https://artfulrobot.uk/civicrm-custom-birthdays-report

https://sf2012.civicrm.org/sites/default/files/slides/CiviCRM%20Reports-Customize%20and%20Extend.pdf

The basic steps are, create .tpl and .php files based on a report that is close to you what you want (mailing details in this case), add the little chunk of code into the .php file under "columns" section and under "filters" section as detailed in the resources above, drop these two files into your websites custom templates and custom php directories, and register them with civicrm.