Possible Shortcut?
 

Is it possible to make a shortcut to a terminal on the keyboard? [ Shift + ~ ] ; if a terminal window is open at the time on the same desktop then it should switch focus to it [ Shift + ~ ]; if a terminal window is not open on the same desktop then it should switch the terminal to the current desktop and change focus to the terminal [ Shift + ~ ]

I wouldn't think so ...

Shft + something is a bad choice, btw ... you
should consider Ctrl or Alt ...

And even though most DE's (and many window
managers) will allow you to define hotkeys I don't
think that any of them will allows scripting (which
is what you require) :}

So: Open a terminal with a hotkey, yes, be taken
to the same terminal with the same hotkey, no.

Cheers,
Tink