Horrible choice to backpass by Jones there, yet again United thankful they have De Gea in goal and his great feet.
Rest of the law? That's in the bit i quoted, as was the fact that the Watford scorers left leg was closer to the goal than the ball and 2 liverpool defenders, therefor offside.
No it doesn't. Probably 99% of the time it does work out that way that the ball is played forward to an offside player in general play. But it's still possible for a ball to be played backward and a player to be offside, direction of the ball is not mentioned in the law.