How to illuminate a certain area in a 3d space

The easiest way to control the way lights affect a 3D layer is to select the layer and press 'aa' to reveal material options. You can also add ambient light and adjust how ambient light affects a 3D layer.  This is a Spotlight, two identical layers, then adding ambient and making adjustments.

You can't. not directly in the scene. You need to somehow splice out that extrusion face and treat it in a separate comp, then overlay it back onto your other scene.

Mylenium