1. What does this even mean? IE9 uses whatever codeccs the user installs
2. It is, ironically more so than others who have adopted HTML5 features that aren't yet standard which could lead to interopability issues when those features do get standardized due to different implementations.
3. customizable gui for what? the point of IE9 is to be out of the way of the web. wouldn't be surprised though if a 3rd party offers some sort of skinning but i doubt that is on the radar for MS.