If a batter goes to pull out of their stance as the bowler goes into their delivery stride but still plays a shot is it right for the umpire to call a dead ball?

As long as the batter satisfies the umpire that they are not ready (before they receive the ball), then the umpire should call and signal dead ball. The batter is therefore immune from being dismissed.

If however, the batter plays a shot at the ball (after pulling away) the umpire will not call dead ball as the batter has brought themselves back into the game - they can therefore score runs of that delivery and also be dismissed.