Basically to make the new arms move like the player arms, I attached them to the body and then made their x rotation copy that of the opposite leg. That way the arms only move when the mob moves and rotate in the same way.
This is absolutely fantastic work! I know you said that you can't have steve and alex in the same pack, but would it be too much to ask for you to make it optional for the pack user to pick which one they want (i.e. you make an additional pack for alex, or just provide the models to enter into the existing pack)?