What the? If you know you're going to cancel do not take the test. How is that even a question?
If it's past the refund deadline (not sure if it is or not) then you might as well, unless it's especially negative for a cancellation or low score to be on your report. Might as well get your money's worth. It can also be worth it to get the "test day experience" if it's already a sunken cost.
That said, OP, you'd probably be fine with a cancel on your report, especially if there aren't any other cancellations or scores beyond October. Basically you don't want it to look like you have a history of canceling/not showing up/performing poor. Unless you're likely aiming for the very highest ranked law schools (as in, top 3 or so) where they're really splitting hairs, it seems practically insignificant. Also, schools only report the highest scores of applicants for US News, so they really don't have much motivation to care beyond that.
I don't think you'd need to address a single cancellation in an addendum.