1. Yes, usually the "best" is a matter of preference. You always should start with the country and then language if you go down that route so that the first subfolder is the geotargeted folder. But other than that, there is no real "right" answer.
2. UK vs GB is because GB is a part of the UK. The recognized country is the UK so that is used for geo-location. GB is the region in the UK that speaks a recognized version of English. Make sense?
3. Yes, companies use languages when they are going to geo-target their company or material for specific countries and then offer translations within those countries. If you are only operating in the US in English, then you don't really need all of this.
However, it won't hurt to set it up for future structuring.
