YITH Topbar Countdown allows you to show a banner in the top of the screen with a simple countdown for offers and promotions.
Yes, the page is a simple template and you can override it by putting the file template "tcountdown.php" inside the theme folder. You can also customize the style by adding your custom CSS in the specific option of the settings.
Yes simply leave the "Link" field label empty to achieve this.
No, you can deactivate the topbar countdown even if the plugin is active.