I also noticed in your picture that the ball is facing up already before you get to the "high T" position. This could be the problem that is causing you to drag your arm. You may have fixed this already but this will cause your arm to stop like you said before. Try to move your arm in your normal arm path once with your hand facing down and again with your hand facing up. You should feel the strain that this hand placement puts on your shoulder.
Back to your original cause for this post. One way to keep your head straight is to think as if the button on your hat is connected to a clothes line that leads right to the catchers chest. If you can keep your head from slipping to the left, you will definitely have more command. Try not to let your head drop either, after you have released the ball, keep your eyes on the target the whole time.
One drill that you can do to help correct "spinning off" and/or "leaning" is to place your back foot on a chair, bench, or bucket. Start with your feet spread to about 75% of your stride length. Start with your hands in the set position, rock back a little bit to gather yourself and throw the ball to a squatting partner. As you throw the ball, make sure that you are accelerating late, throwing the ball down, and most of all bending your back. After releasing the ball, you should have the same ankle roll that you would on a mound. After your ankle rolls, keep your back foot, toes down, placed on whatever apparatus you choose to hold your foot. This drill can be done anywhere from 30 to 70 feet.