How to pause Full Screen Preview? & Other controls...

Oh wow, I forgot all about that method. I don't think I've touched it in years! I just tested it and when I start the preview it goes full screen, and when I stop it exits full screen. This makes sense because otherwise you have no control of how to escape full screen. I don't like it, so I don't use it, but I'll either use Mercury Transmit to a second monitor or maximize the Comp panel with the Tilde (`) key when I want full res previews. If you maximize a panel make sure your mouse is hovering over it first, otherwise it will maximize the panel under your pointer. 

Yup, that's the way to select it. I guess the first question is, how are you going full screen? Are you hovering over the Comp panel and hitting the Tilde (`) key, or are you using Mercury Transmit and using the forward slash (/) on your NumPad to trigger full screen?

If the Timeline was selected when you went full screen then the Spacebar should pause. As for moving forward and back one frame, you can use the Page Up and Page Down or Ctrl + Left Arrow and Ctrl + Right Arrow. Both shortcuts do the same thing.