Now, let me explain this bit for you. So, if Farrell was in the process of going to make a legal tackle and then suddenly there is a change of direction or a significant drop in height from the ball carrier (either voluntarily or due to another player) which caused Farrell's tackle to become illegal, then the referee would apply mitigation. In that situation, there would be no way he could have changed what he was doing.
The key thing about mitigation here is that Farrell's actions were always illegal. No matter what happened, he was always leading with the shoulder and not making an attempt to wrap for a legal tackle - so mitigation does not and should not play a part in the decision.
So, there is no mitigation for me, and there is certainly not enough to mitigate down from a red card, as it is foul play, there is a high degree of danger and there is an act of illegal ness which is leading with the shoulder.