If no widgets are showing in the right sidebar on your Category view of posts, drag a widget into the “Blog Right Sidebar” position. As for wanting a full width without sidebars on the left or right, this is not possible with this theme.
For number of posts to display per page, you set this from your WordPress Settings >> Reading
For changing the url, this will require changing the permalinks or using a plugin for doing this kind of custom url structure.
