I found a way to activate the internal monitor while no display is active.
Following instructions here and upgrading to Xfce 4.12, there is a new window for
xfce4-display-settings -m which by default is already activated by pressing Superkey-P. (Logout and in after upgrade.)
Even if the monitor is still closed, press Tab (three times in my case), and then Enter, but pressing one or two times and Enter would still activate internal display.
Even easier: instead of Tab, use right-arrow multiple times and then Enter: the internal display is the last button to the right and it stays selected in this way.
But in fact this might have worked even before the upgrade with just the advanced window (
xfce4-display-settings; before upgrade, adding
-m displays the same window as far as I can tell) by pressing down-arrow key, then Tab (once) and Enter.
But the upgraded version is more interesting, more easy to use and has enhanced features (extended display).
After vasa1's answer to a different question, a solution emerged that in the end may be preferable, as it involves single, separate, clear shortcuts for each display configuration.
But this involves LXRandr, the LXDE default display manager, which can be installed from repos and used perfectly under the Xfce desktop. It is a GUI for
xrandr and saves its configurations.
Copy the executive command (it would be something like
xrandr --output LVDS --mode 1680x1050 --rate 60.1 --output VGA-0 --off)
In Xfce, under Menu, Settings/Keyboard/Application shortcut, add this command and the shortcut.
This procedure can be repeated for the other two options (external monitor only, or mirror image - but there's no dual display in lxrandr).