Aaron
Does anyone know if there is a shortcut key combo?
For what exactly?
There certainly isn't one for enabling Fast User Switching, or for going to the Login Window from when you're logged in (except Shift-Command-Q, but that will actually log you out, which is not what you want). You need to select a single item from the "Users" menu.
To set up the screensaver and enable "hot corners" also requires you to visit System Preferences > Desktop & Screensaver and choose "Hot Corners …" on the Screen Saver tab. Then you need to visit System Preferences > Security and check the box for "Require Password to wake this computer from sleep or screen saver", as I said before. There is no keyboard shortcut for any of these setup operations, and sadly, once you've done that, you still need a (single) mouse click in one of the bottom corners to turn the screensaver on.
So, sorry, but you will need your mouse! Although I've no doubt something could be set up in Automator 🙂