Rhetorical situation- is when a situation is created by writing or language.  The only time I had ever heard someone use the word rhetorical is when they say they are being rhetorical.
Discourse-Discourse is when writing or language motivation action. An example of this would be a speech or some other type of persuasive writing.
Audience-I think the audience is anyone that reads or is meant to read your prose. Grant-Davie uses the example of a Coca-Cola advertisement but I think that any type of writing has a audience that it is meant for.
Exigence- I think exigence is anything that communication can solve.  I thought of this as something as basic as how people use words to communicate.
Constraints-I believe that constraints are anything that limit the solution to the problem.   An example of a constraint would be not wanting to cause a side effect.