I've been thinking and I reread the last two pages of this thread.
I'm going to compare the Red Wedding to the sack of King's Landing by the Lannisters. Well, just insofar as who actually did the nasty work of the killing of important persons...
At KL, it was chaos. For Robert to be secure on the throne all the Targs had to die. But getting inside KL and the Red Keep would require a major fight or betrayal. The Lannisters tricked the Targs and turned on them once inside... the biggest betrayal was that of Jaime Lannister killing the king he swore to protect. Fast forward seventeen years or so, the Martells are still in the dark about who actually killed Elia and her children. Sure they've heard the rumors, but they'll either have to settle for the justice that Tywin (the orchestrator of the slaughter) doles out to them or they'll have to fight Tywin (not a pretty prospect).
At the Twins, it was tense. For Joffrey to be secure on the throne all the Starks had to die. But getting close to Robb would require a major fight (and the Lannisters had lost every time to him) or betrayal. The Lannisters bought the Freys and the Boltons into murdering Robb at a wedding... the Freys broke a major taboo by killing their guests and Roose Bolton broke a major taboo by killing his liege lord. If we could fast forward seventeen years, we might find Robb's son, Sansa, and Jon demanding justice from Tommen, Roose and Tyrion, but they'll have to settle for the Freys as scapegoats for the whole thing... because Roose will say "I was not there, I was fighting the Lannisters... Only after Robb's death did I take up leadership in the North... all I did was for the realm." And Tommen will say "Lord Tywin in no way was connected to the Freys. Lord Walder did all that on his own initiative out of a desire for vengeance and a way to ingratiate himself to the throne... he hoped to be made Lord of the Riverlands. And Robb's son, Brynden, Jon, Sansa, and the rest will either have to swallow that load of crap or go to war.
So, I think Lord Tywin purposefully let the Freys play the part of Jaime, i.e. caught red handed. But just like the Mountain's and Amory Lorch's parts in the sack of KL, Roose Bolton's and the Lannister's parts could be clouded up and covered up over the coming years.
I do find it interesting that when Roose invited Brienne and Jaime to sup with him in Harrenhal we had the three known Kingslayers sharing a meal.