When should I use format type:
2) “xs:duration” ?
Well, it always depends on your application
It seems to me that you mainly have more general questions about the XML Schema built-in-datatypes.
So the link below might be helpful for you.
Here you will a find a graph, showing the inheritance structure of XML Schema built-in datatypes.
If you click on a certain datatype, you will get more information about value space, constraining facets and also sample values.