Suppose I embed a Visual Force page in a standard layout and write a controller extention based on the controller on the same object as what the page layout is based.  For example, embed a Visual Force page in the Display page for Opportunity and write a class that extends the Opportunity controller. I'm was hoping to use standard control for the rest of the layout and extend the standard control for a few functions on my VF page but I still want to work in the context of the object on the page layout. I want to be able to pass data back and forth.

If I extend the Opportunity controller, do I get a new instance of the Opportunity controller that isn't the one control the regular page layout?

Do I get a different instance of the controller? (if that even makes sense, does a controller also drive standard layouts)

Is there anyway in my controller extention to refer back to the page layout?  What I really will want to do is rerender part, or maybe all, of the page.