would this be something that can be used on installed windows? If not what is a product that can be applied to windows that can not be scratched off?

Yes, the cream material can be used on a window to frost etch the surface permanently. It's more difficult to do a larger area with the cream, as you only want to have the material on the glass for about four minutes for a clean etch, and as you start applying the cream to the glass, it will start etching immediately, so the first area you apply may already be at four minutes by the time you finish so it can be difficult to get an even etch on a large surface.