The two piece trigger allows the safety to work. When the bottom piece of the trigger is against the safety the top piece is free to move forward to allow the half cock and full cock notch to go past it. Some folks weld the trigger parts together to make it a one piece trigger. I don't recommend doing that.
It would have been just a miss for the unfired pistol round. After firing the 10th pistol round after the shotgun was used ( gun shot out of order ) the call should have been just a procedural as long as the last pistol round hit a pistol target. By the way if you had redrawned the first pistol and fired the 10th round before using the shotgun there would not have been any penalty.