I love the analysis (and, bizarrely enough, was looking for the proper way to address an Emperor on a completely unrelated matter) .... but, alas, I don't think any of it entered into the thoughts behind the screenplay. I simply think the much simpler "Your Highness" was easier on the tongue, easier on the minds of the audience, and a merely expedient way to convey what the script wanted to convey - without doing somersalts of language.