Don't cancel. It's not remotely worth it to cancel and getting a mediocre score might help focus you so you don't take it unprepared again. You're likely writing an addendum either way so you may as well see the score.
I've never heard of this from US law schools, and I've seen the applications for when I intended to apply for the 2016 cycle. If anything, this would be addressed in the addendum.
No, do not write an addendum for this. You'll just be drawing way too much undue attention for a minuscule difference. It will just be another thing for an adcom to read without adding value which is something to avoid.
I don't ... :
The point of an addendum is to show your good ... tell you to submit an addendum for anything; I've come ... believe that a low-LSAT addendum is just as likely to ...