I've stopped using mustard because I don't think it makes a difference, for me rub tends to stick fine to the surface of the meat. I would also say that brisket doesn't traditionally have a sweet element to it (the brown sugar you added). That said, if you like mustard or sugar with your brisket rub, GO FOR IT, because YOU are the one eating it!
Did you separate the flat and the point before carving? That can help determine which way the grain is running. It's also a good idea to mark or notch an end of the brisket before putting on the rub and cooking, because it's much easier to see the grain in the meat before the bark forms. That way when you pull it off you simply find your notch and go from there.
Welcome to the club! You'll learn a lot here.