I am using Ubuntu QQ with
gnome-session-fallback for a long time now. I replaced Nautilus with Thunar but when I restarted the laptop, the panel works, but I cant right click or change the desktop background.
If you replaced Nautilus with Thunar (How?), there is no desktop because Nautilus draws the desktop, so if it's not running you don't have a desktop.
So run it (you will get a Nautilus window though). Or, use