The inner pane is typically half the thickness of the outer pane.
So whilst you could argue it cold be seen as a failsafe, I would say its primary purpose remains "double glazing" insulation.
The OAT at 30–40,000ft is very low and there is a lot of combined window area. So the importance of insulation is not to be underestimated. In particular on newer aircraft where efficiency is the name of the game in their design.
That's the scratch pane you are referring to.
Yes, it is made of cheap plastic and serves no structural or other purpose other than to protect the real stuff from annoying kids. ;)
Depends on the aircraft but a typical design would connect the space between the panels to an air supply or otherwise a self-contained desiccator system.
Flight deck windows are completely different, typically three layers, two full thickness and one half thickness. All fully heat/chemical strengthened with additional anti-fog, anti-ice and moisture absorption systems built-in. The fact that some of them are sliding (i.e. openable) adds to the design complexity.
For the non-aviation folks, OAT means "Outside Air Temperature".
The inner cabin window is absolutely there to protect the hull window.
