To make the main page that comes up when some one enters your domain, the page you want to come up will need to be named index.html. For more info on this please see: http://www.aota.net/HTML/homepage.php3
Yes you may alter pages with the editor in the CNC
People cannot alter your source code on the server unless they have your login information. They may of course view it and change it on their own computer, but they cannot affect what’s on the server without your login information. In regards to password protecting your directories this is only needed if you want to only allow certain people via their browsers to view a page/ directory etc.
If you have any more questions feel free to ask away