Landing on the heel of the front foot is also something I had a problem with. As you say, it all ties in with the centre of balance not being in the right place. Most pro legspinners will usually land on the ball of their right foot, without the heel having any weight on it, and then land on the ball of their left foot, again, with all the weight being on the ball of that left foot. One thing I did to help with the downward swinging direction of the bowling arm was to have the arm closer to my torso as I brought the arm down. Previously, my arm would be quite a way outside my body. As you say, you generate more energy in the arm if it is closer to the body so it is natural for the arm to then swing back towards the body if it is quite wide - even more so if the centre of balance is a bit off.