1) You may look for third-party plugin or copy the xstore/404.php file to child theme and edit its content according to your needs.
2) Because that is 404 is for pages http://prntscr.com/oozj28 and by default search is for pages/posts, not for products. If you want to get search by product there you can replace get_search_form( true ); by etheme_search_form( array( 'action' => 'default' )); http://prntscr.com/oozr6t
