Make windows go completely off screen in “show desktop” hot corner

You can directly copy to/from the desktop to/from fullscreen mode using drag-and-drop. You need to go via the desktop "preview" window of mission control (at the top):

Here's a video demonstration.

The purpose of the shadows is to show that you are temporarily pushing the content to the edges to "clear some space" over your desktop, presumably where you will then copy or rename a file or perform some other short action with one of your Desktop items. It's existence signifies that you have plenty of windows open and ready to return to their previous on screen location at a gesture.

If you removed it, then it would look no different from a normal desktop with all apps minimised, or say a fresh desktop space within Mission Control, and how then would you be able to distinguish between the 2?

Obviously the above isn't necessarily an answer to your question (my answer would be "There is no supported method of doing this nor am I aware of any defaults write style .plist edit that would hack it), but perhaps understanding the purpose of them more will enable you to put up with them better.