RE, the line tool, you can start with the example line tool ("linetool.rb") in the “Example Ruby Scripts” extension, by the SketchUp Team.
I just posted an example in this category day before last:
But, … it would be best not to actually create temporary edges and faces, but instead just write a 3 click tool that draws a virtual 3D box on the screen, and then after the 3rd click it creates the dynamic component.
This way, you will not need the observer kludge.