Howdy,
As wjosten noted in your first link, there is another place to configure "Show Desktop",
> System Preferences > Mission Control > Show Desktop
Check and see if that is set for "Right Shift". If so, change it to F11.
If both settings look like they are set for F11, change the setting in
System Preferences > Keyboard > Shortcuts > Mission Control > Mission Control [aka your screenshot] to something else, like Command-Shift-D and test, then change it back to F11 if you want.
That toggles the setting to force a change.
There is also the "Restore Defaults" button in the Shortcuts window, but that would reset all the defaults which may not be desired.
ivan