"Do you think of that as an authoratative statement about the truth of the character, or an interpretation of the character as you have perceived her in play?"
Intriguing question and one that I'm not sure I know how to answer.
It is an interpretation of the character as I have perceived her in play -- but I have a privileged perspective which I consider authoritative. To borrow your "final authority" test, if someone were to say to me, "Sarah still isn't committed to Lucas," I would say, "No, you're wrong." Not, "No, I don't agree," but, "No, you're wrong." I know how Sarah feels. Now that might well change over time depending on what happens in play. But it doesn't mean I necessarily know how her feelings will change, or she will act. Case in point: Sarah's intent in approaching Aaron at the party was to close off the whole will-we-won't-we episode in an amicable and grown-up way. If you had told me that Aaron was going to proposition her, I would have said that she'd try to turn him down. Her honest intent in that scene was to clean up the mess she had left, and walk away. I was surprised and horrified to hear her say yes.
Does that answer your question? I guess part of what I am trying to say is that I think you're putting up a false dichotomy: I can have an authoritative understanding of my character which is discovered rather than predefined. But maybe I'm also backing up your dichotomy, because my authoritative understanding is restricted to interpretation of what has happened: I can make authoritative interpretations, but not authoritative predictions. And -- excuse my presumption -- I suspect that this is true of you and Aaron as well: you know his mind in retrospect, but not in prospect. If that's true, I come back to the difference of degree: concerning knowing our characters' minds in prospect, you may have no expectations, I may have unreliable expectations, and strawman-version-of-Vic may have completely reliable expectations. (Since Vic has mentioned that it sometimes takes her a week to distil down Rachel's mental state, I suspect that in reality Rachel is not as fixed as Vic makes out!)
I'll throw in a further wrinkle. Having insisted that my experience of the character is authoritative, I have to admit that I can be wrong. My initial interpretation of Sarah and Aaron was like yours: that we were back to square one. But as soon as I started thinking about possible scenes for the next session, I knew that initial interpretation was wrong. So maybe my "authoritative" understanding is not so authoritative after all. I guess we'll find out on Thursday...