For a start see Including files in the Tutorials section.

In general use modules to organise code into libraries (collections of related code). One reason to put code into modules is so that you can write test suites that focus on the interfaces provided by the modules. That allows you to develop module code independently of the client code with some assurance that you haven't broken the client code that depends on it - if the tests still pass the client code ought be happy with the module changes you've made.

Pretty much the answers to all your numbered questions fall out of considering why and how you want to organise the code into modules (except 4, to which the answer is 'No' and maybe 2, to which the answer is 'Yes').