Thanks for reaching out. You can find a Custom CSS snippet here for a full-width header. Instructions for where to insert Custom CSS snippets can be found at the top of that page.
You can add a widget area to the end of the navigation using https://wordpress.org/plugins/wp-widget-in-navigation/. See instructions for usage on the plugin page. You can add the following to Appearance > Custom CSS or Customize > Additional CSS to improve the alignment of the widget area: