There are two easy options.
Ctrl + right click on plank and go to preferences. There is a show desktop docklet.
Hot corner solution. Install wmctrl
sudo apt install wmctrl
Then you can use wmctrl -k on in the custom command box.
It seems that as of now, elementary designs hot-corners to work like wingpanel and the dock and show them only on the primary display.
There is a way and it is actually has been explained in I believe /org/pantheon/desktop/behavior which is seems like settings area that we can access with dconf-editor (or you can see .xml version of it at /usr/share/glib-2.0/schemas/org.pantheon.desktop.gala.gschema.xml)
If you just put a single command, it will be used for every hotcorner
As far as I know, there is no way to run a command after leaving a hot corner, but you can toggle "Show Desktop" when you touch a hot corner.
I've got the following script from this article:
You first need to install wmctrl (sudo apt install wmctrl)
Save the script:
status="$(wmctrl -m | grep "showing the desktop" | sed -r 's/(.*)(ON|OFF)/\2/g'...
I don't think there is a possibility to trigger selectively. However as a workaround, you could use a custom command that would start by checking if the alt key is held down and trigger the action if and only if…
There's an "elementary" Solution for it (pun intended)
Keep pressed CTRL
Right Click anywhere over plank
Click on Preferences
Click on Docklets
Drag the Desktop icon into plank
Docklets in Spanish was translated as Agregados, a really bad translation must say. Better to keep it as Docklets
I came looking for a solution to showing the desktop in a single key press and the solution of mapping wmctrl -k on to SUPER + D got the job done. Quite handy if you have a really funky conky monitoring configuration running on the desktop.
Note that the above seems to be a half solution though.
If you want to restore the windows you were working on ...
As far as I know, there isn't currently a way to add titles to multitasking view.
If you'd like, you can file a feature request on Gala, the elementary OS window manager. For more on filing bugs and feature requests, see this question.