Questions 1 & 2 seem to relate more to using separate windows.  Take a look at ControlP5 library for processing.  You're going to have to use a buffered image if you want to reference a zoomed in window.

As far as using +/- for control of the zoom level (as opposed to using something like, say PeasyCam), there are several methods to perform actions on key events.  I _think_ the keycode method is the most portable and reliable method but I could be mistaken.

1. void keyPressed() {
2.   if (keyCode == KeyEvent.VK_MINUS) {
3.    DoYourZoomOut();
   
4.   }
5.   if (keyCode == KeyEvent.VK_PLUS) {
6.   DoYourZoomIn();
7.   }
   
8. }

See http://www.processing.org/reference/keyCode.html