One of your original questions, why did they need a decoy?
1) She was obviously the leader. At least in the full version
2) The robbers couldn't just stand out in the open so they needed to be well hidden and 3) her part was as a distraction and delaying tactic. The 3 butchers were initially riding 'as fast as they could trig' so would have been difficult for the robbers to ambush.
4) They also gambled that if Johnson was chivalrous enough to rescue the girl, he would also want to protect her when they started the ambush.