Sepinwall also notes that another critic mentioned the possibility of Jimmy editing as he went along, to spare Kim what Chuck really had to say, which hadn't occurred to me. The more I think about it, the more I agree that, if you only look at that last scene, it's probably Chuck wrote the letter back in the day, when he could still feel expansively, patronizingly superior to Jimmy...
...but the stop at the courthouse still gives me pause. Could be a red herring; could just be designed to show that Viola's a little bit of an intrusive keener; could be something. I'll say this for Gilligan & Co., they don't tend to leave this stuff ambiguous.