1) The history is deleted when the workflow is deleted. They go together.
2) The best way to document it is to associate it to the document that was used to start the workflow. You can include some actions in JS associated to the transitions between workflow states. I did it in a very complex workflow (+50 states) and it works OK.
Note: the workflow is temporary while the documents are "permanent", therefore, if you want to make permanent workflow information, you have to extract it and copy somewhere else.
Note2: JPotts has a sample on how to define actions in JS. Just add code (e.g. logger) to extract and copy the workflow information somewere else.