How to keep "active" css on menu item when clicking on other link on same page?
You could consider using jQuery client-side to set the class based on path. See this post on how to do that: http://www.itworld.com/article/2832973/development/setting-an-active-menu-item-based-on-the-current-url-with-jquery.html

or you could try using Menu Trail By Path module
https://www.drupal.org/project/menu_trail_by_path