Vers, I have watched Mason a great deal and the problems i see with his acuracy is direct;y tied to inconsistency in his feet. upper body mechanics, I really like a lot. he keeps the ball high just below his ear, he doesnt have the big windup but just rolls his shoulder and it comes out beautifully.
If you watch his feet however, when he throws he throws with a short stance and doesnt drive his hips with the throw. I believe this can be solved. Its why he wont go in the first but I see a steal with this kid in the 2nd round.
He just has to learn to dance. its not that he goes to a wide stance but he short steps or doesnt step and thats horrible for accuracy but correctable.