The plugin installed on your site can be found on Admin > Plugins > Installed Plugins. Visual Composer and Cornerstone is both a page/content builder. It will depend on you which do you prefer to use. Cornerstone is the page builder that comes with X theme. It is a required plugin, whether you want to use it or not, it is installed automatically. On the other hand, Visual Composer is a bundled plugin, another option to build content.
Correct, feel free to edit the demo page/post to build you own content or delete it completely and create a new page or post. Whatever works for you is fine.
Customizer/Theme Options, is where you will set sitewide settings for your site. This settings works for every page on your site or as stated on the options.
For more information, feel free to check our knowledge base here.
Hope this helps.