1: Roughly, window manager is lower layer than "desktop". WM manages specifically drawing windows, their widgets, borders etc. "Desktop" is a whole enviroment - windows, panels, menus, applets... KDE and Gnome are desktops. Enlightenment is also a desktop, but not as all-encompassing as the other two.
2: Enlightenment runs instead KDE/Gnome, not "on top of".
3: Pass - that's Red Hat specific stuff
4: Depends on how you start your X sessions. If you use KDM/GDM you'll need to make an entry for Enlightenment in their configurations - see KDM (or GDM). If you use "startx", add a line "exec enlightenment" or something close to that to your ~/.xinitrc