Yes. The Website Builder tool allows you to build what are known as "Responsive Websites" or "Mobile Friendly Websites", which means that they will detect which type of device your visitors are viewing your site with & it will present them the most suited website format.
Yes, absolutely. You are 100% in control & you can make as many changes as often as you like.
Whether you want to add pages, remove pages, change an image or amend the text on your site you just need to log in, make the change & publish to your website. It really is that easy!