I wonder if it makes more sense to call gnome-shell a configurable, feature-rich "addon" to a basic gnome desktop ( which without the shell would have minimal or no user functionality), so, perhaps the answer to q3 (gnome-shell without a desktop) is that it doesn't make sense, as its a plugin to the desktop?
It sounds like it is a fundamental mandatory component to the gnome desktop. Without the gnome shell, I'm guessing a user would not be able to do anything at all, apart from click on any icons/apps that happen to be already sitting on the desktop.
TBH, the wiki page is actually quite rich in it's definition which I have copied over to here below.
The GNOME Shell comprises the following graphical and functional elements:
System status area
Notifications and calendar tray
Indicators tray (deprecated)
GNOME Shell is tightly integrated with Mutter, a compositing window manager and Wayland compositor.
Changes to the user interface (UI) include, but are not limited to:
Clutter and Mutter support multi-touch gestures.
Support for HiDPI monitors.
A new Activities overview, which houses:
A dock (called "Dash") for quickly switching between and launching applications
A window picker, similar to macOS's Mission Control, also incorporating a workspace switcher/manager
An application picker
"Snapping" windows to screen borders to make them fill up a half of the screen or the whole screen
A single window button by default, Close, instead of three (configurable). Minimization has been removed due to the lack of a panel to minimize to, in favor of workspace window management. Maximization can be accomplished using the afore-mentioned window snapping, or by double-clicking the window title bar.
A fallback mode is offered in versions 3.0–3.6 for those without hardware acceleration which offers the GNOME Panel desktop. This mode can also be toggled through the System Settings menu. GNOME 3.8 removed the fallback mode and replaced it with GNOME Shell extensions that offer a more traditional look and feel.