(Disclaimer- I look at this from a different angle as South Africa uses Roman-Dutch law, which differ slightly from the common law which are applied in these cases)
I found the reasoned decision quite interesting, but would have handled the appeal differently if it was my case.
The appeal only dealt with the first ruling and no new evidence was presented. The IRB argued -
1 That the JO confused the charge by referring to the law related to legitimate rucking.
2 The JO applied the wrong test in terms of burden of proof, beyond a reasonable doubt instead of on the balance of probabilities.
3 The JO made a manifest error in his factual findings.
Point three was a hail Mary call and doesn't warrant discussion and was rightly ignored by all the parties.
If I argued the appeal on behalf of the IRB, point number 2 would have been my main focus. The JO used words to the effect that Horwill's version was "not impossible or even highly improbable". These words are what you expect to find in cases requiring proof beyond a reasonable doubt. The correct question that needed to be answered, which wasn't, is whether Horwill's version was in fact the most probable (or at least as probable) version based on the evidence as there were only two possible versions presented at the hearing*, +.
Mew did some interesting mental gymnastics to argue that the Hampton did in fact apply the correct test, but does not really give any solid reasons to negative the IRB's argument.
The first argument is also an interesting one. Hampton referred to a 2005 clarification to point out that, in the context of rucking, contact with the head will not automatically lead to finding of guilty unless intent or recklessness can be proven. It is pretty clear why Hampton included this in his finding. The point has been raised in this very thread already with posters arguing that all contact with the head should result in some sanction being imposed. The point Hampton was making was that, in terms of the laws of the game, boots and heads will sometimes come into contact with each other without a player being guilty of an offence.
The point the IRB argued (which Horwill conceded and Hampton accepted) was that this case did not deal with rucking for the ball. The point that Hampton failed to deal with, which goes to the heart of the matter for those that believe Horwill is guilty, was that the ruling on rucking is manifestly different to the current case. Where a player is rucking for the ball, there is a legitimate (legal) reason for his movements. For those that believe Horwill was guilty of stamping, there was no legitimate reason for him putting his foot down where he did. More to the point, his actions seem so unnatural that one has to come to the conclusion that he intended to stamp on Alun Wyn Jones.
Mew correctly found that Hampton made his comment in a specific context and Hampton's failure to deal with the other issues raised does not mean that misunderstood the charge against Horwill.
It might be different in common law jurisdictions, but here one often look at the cost order to see who in fact won a case as the "winner" usually receiving cost order in his favour. In this case each party was ordered to pay their own costs. Mew is at pains to point out that, though unsuccessful, the appeal did have merit and that the IRB were right to appeal the original decision.
* It would have been interesting had Mew found for the IRB on this point. I wonder whether the case would have been retried or whether the case would have been referred back to Hampton to rule on the facts, applying a different (the correct) test.
+Based on the video we have all seen I am of the opinion that Horwill's version is not the version best which correlates with the available evidence.
