(a) The US and its various territories is the only country that still uses Fahrenheit. There was an effort years ago to move the US to the metric system that flopped. Sooner or later we'll have to bite the bullet. In the meantime we still sell stuff by the pound in the grocery store, gasoline by the gallon, and 1/4 inch socket sets. :-(
(b) The truth is that absolute zero on the kelvin scale is an abstraction. The wonder is exactly what would happen if you could get to that temperature. Thus a great deal of effort has been spent trying to get to low temperatures to test the scientific theories about the interaction between matter and temperature. At that sort of low temperature weird quantum mechanical effects are observed, for instance Bose–Einstein condensates. See the Wikipedia article on Absolute Zero for more information.
Also because of the way our scientific theories are axiomized you can have negative Kelvin temperatures. Negative temperatures are strictly a quantum phenomenon.