1: Yes, you can either use sections to dividede your site or other methods (subree three only available to some users for example). I have made a site with 6 different user groups with different access, not very difficult.
2: I do not know.
3: Yes that is be possible, and not to difficult. But little documentation on how to do it, you would have to play around with the system to learn how to do it.
EZ is very flexible cms. However, everything is not so very well documented meaning that lots of advanced stuff is possible in a theoretical sense. However, in pratical senses it can be difficult.
In regards to question 2, is it the admin interface that is a problem or is it the potential to work on other parts of the site than the guestbook? If you don't mind the admin interface, you can allow users only the ability to edit/publish certain content types (such as a guestbook entry) within the admin interface. They could see the rest of the content the same as if they were on the user site, just with the admin templates. You can disable the access to any setup functions, to adding content, or to seeing user accounts.
Also, I think that you can override the admin interface to make it look any way you want (i.e. like your user site), but I am not sure if you would want to try that, as it seems like it would be a lot of work.
In regards to question 3, you can use the same design folder for many sites without any trouble. Each site would be listed in the site.ini.append.php and each would have its own settings folder. However, the site.ini.append for each site would list the design as being the same. You would have to duplicate the override settings for each of the sites so that they would use the same templates.
