How can I clear scrollback and reset with a keyboard shortcut, rather than the mouse, in xfce4-terminal?

You can custom configure hot keys via config file:

nano ~/.config/xfce4/terminal/accels.scm

Find and replace for set up hot key Ctrl+Shift+l Clear Scrollback and Reset:

(gtk_accel_path "<Actions>/terminal-window/reset-and-clear" "<Primary><Shift>l")

To Clear Scrollback and Reset in xfce4-terminal without using the mouse, use the keyboard combination Alt+t+c.

Note that this can only be done when the xfce4-terminal menu is visible. To toggle the menu, press F10.