Yup, nice explanations, Schoolmarm.
However, regarding essential and non-essential clauses, surely the context in which the sentence appears must have a bearing on what sort of clause we're talking about? I only say this because of your examples:
"Eleanor of Aquitaine was a powerful woman who married two kings during her life."
The sentence still makes sense and conveys a lot of information if you remove the middle clause:
"Eleanor of Aquitaine married two kings during her life."
Is the sentence trying to say that she was a powerful woman
because she married two kings? If so, then wouldn't this information be better conveyed by saying "Eleanor of Aquitaine was a powerful woman because she married two kings during her life."
If not, why is the 'powerful' woman clause essential to the sentence? Couldn't it be a separate sentence all on its own? "Eleanor of Aquitaine married two kings during her life. She was also a powerful woman." Why is the clause any more essential than your second example:
"The movie Empire of the Sun, which I did not see, was based on a true story."
I think I could make a reasonable case that the middle clause is just as necessary or unnecessary as the first example. If the context of the discussion in which the above sentence appeared was about movies which people did or didn't see, and whether or not those movies were based on true stories or not, then surely that middle clause is pretty essential?
Yvonne
(confused
)
PS: Oops, just read your caveat about ambiguity. I guess the above comes into this category.
