Adding expired/grace membership numbers to the Membership Dashboard
In my experience, the Membership Dashboard isn't really a great tool, and I was it was deprecated.  It's extremely inflexible (as you're seeing here) and any reports you need can be placed on the main dashboard as a dashlet.  So I would recommend a different approach.

To answer some of your other questions - class names and file locations map to each other.  You should find the CRM_Utils_Array class in &lt;civiroot&gt;/CRM/Utils/Array.php.

Finally, it's a bad idea to change core CiviCRM files, because they'll be overwritten on upgrade.