Ref Beirne's red, this is how I see it IMHO:
Was the decision on the field to have a yellow and review in the bunker? yes. Was the decision to upgrade to a 20 minute red based on the current rules as prescribed? yes. Was it right to rescind the red on review with the panel? yes.
Overall there is no-one to blame here, the rules were followed well and the outcomes correct as they are currently proscribed. Does that mean I agree with them in their current guise? No. I do think that Karl Dixon's "not all head contact is foul play" is correct, and some judgement needs to be exorcised.
As a rugby player though I am surprised that Barrett and Beirne seem to of forgotten a rule that is instilled in all of us the moment we start the game. You always run a line expecting to receive the ball, even if the play is not set up for you to receive it (always run hands up, giving the passer an option as on attack you are always an option!). If someone is running down your channel always be prepared that you will need to make the tackle. Whether intended or not. The moment Barrett received the pass he was in play and fair game and all the rules of tackling applied at that very moment.
If the outcome is that we all remember what we learnt the moment we start of playing rugby, and the rules are adjusted to allow for more in game referee judgement on what is a rugby incident we'll be the richer for it.