Your picking method is inefficient, but not the problem. The confusion is due to the way C2 handles negative speeds in the bullet behavior. When you set a negative speed, C2 rotates the current angle of motion 180 degrees, and changes the speed to a positive value. When you enter a speed of -200, C2 sets the angle of motion to 180 degrees, and the speed to 200, effectively moving the object left. Later, when you want to move that same object right, you set the speed to 200, which continues to move the object left since it's angle of motion is still 180 degrees. The object moving to the left, when your other events expect it to be moving right, is the cause of the ultimate overlap.
The simplest fix would be to add an action to event 6 to set all the card's angle of motion to 0 degrees before setting their speed.