I must say, I'm amazed at the assumption here that a vision Dany saw in the HOTU back in Book 2 must inevitably be fulfilled already. Could the Undying just not see very far into the future?

There are some similarities to the Red Wedding, but saying that this must be a vision of the Red Wedding, just because nothing else has happened as of Book 5 that more closely fits the vision, makes no sense to me. A group of feasters are slaughtered, and a dead wolf-headed king looks on---but we have several potential wolf-headed kings, at least one of whom is in line to be dead yet alive, and the idea of feasters getting killed doesn't strike me as the sort of thing that can only happen on a single occasion. Patchface's prophecy clearly depicts the Red Wedding, because its specific elements align perfectly with what happened in the Red Wedding. But Dany's vision encompasses a number of elements that weren't present at the Red Wedding (an iron crown, the wolf-headed king on a throne who's dead but somehow still alive and watching, the placement of specific foods, etc.), yet ignores many of the defining features that were present at the Red Wedding---the musicians, the dead fool, the crossbow bolts, etc.
Northern, on 23 March 2012 - 08:09 PM, said:
From an earlier chapter, Robb’s crown description:
“Robb reached down with both hands, lifted the heavy bronze-and-iron crown, and set it back atop his head, and suddenly he was a king again. “Lord Rickard dies.”
Is it that much to imagine Dany, in her brief glimpse of the scene only saw the iron portion of the crown? It is not as if she described a completely different type of metal, say a silver or gold crown.
Robb's crown is described as being very distinctive, and GRRM makes sure readers know exactly what it looks like. Catelyn describes it as "an open circlet of hammered bronze incised with the runes of the First Men, surmounted by nine black iron spikes wrought in the shape of longswords." Jaime sees Ryman Frey's "queen of whores" wearing "a circlet of hammered bronze . . . graven with runes and ringed with small black swords." (Whether this is Robb's crown or Jeyne's, the description is too similar to discount.) Brienne later sees Lady Stoneheart holding "a bronze circlet ringed by iron swords." None of these crowns is ever described as "an iron crown", and we are given multiple descriptions that attest to that.
Dany saw enough of this scene to take in the carnage, discern exact food items still clutched in severed hands . . . the idea that she got a "brief glimpse" of the crown isn't supported by the level of detail she goes into. Robb's crown wasn't an "iron" crown, any more than a blue shirt with white trim would be described by anyone as a white shirt. Matching the two crowns requires us to ignore the very distinctive descriptions of Robb's crown, and GRRM tends to give distinctive descriptions to things for good reasons.
Northern, on 23 March 2012 - 08:09 PM, said:
From HotUD: “In a throne above them sat a dead man with the head of a wolf. He wore an iron crown and held a leg of lamb in one hand as a king might hold a scepter, and his eyes followed Dany with mute appeal.”
Robb may not have been sitting on a throne during the feast, but we know there is a throne in the room:
“In the midst of slaughter, the Lord of the Crossing sat on his carved oaken throne, watching greedily.”
Considering what else they did to Robb & GreyWind, it is very possible he was moved to Walder Frey’s own throne, and that they put the leg of lamb into to his hand, as there was lamb served. In fact, both Wendel Manderly & Smalljon were had leg of lamb/mutton in their hand prior to being killed.
“The Smalljon bludgeoned Ser Raymund Frey across the face with a leg of mutton.”
“Ser Wendel Manderly rose ponderously to his feet, holding his leg of lamb. A quarrel went in his open mouth and came out the back of his neck.”
First, I can't see Walder Frey letting anybody, even a corpse, sit on his throne while he's still breathing.
Second, this is actually part of my point. Walder Frey very explicitly presided (literally and metaphorically) over the Red Wedding,
not Robb Stark. Presuming that he must have had Robb's corpse dragged onto his own throne sometime afterward is presuming information that 1) we're never given, and 2) goes against everything we know about Walder Frey (his whole schtick is his refusal to let
anyone onto his seat of power). Wendell Manderly and the Smalljon had a leg of lamb----
never Robb. Yet in Dany's vision, the slaughtered feasters are holding roast fowl, not lamb, and it's the dead king that's holding a leg of lamb.
The wolf-headed king in Dany's vision is dead----yet he's also in some way alive. Though she describes him as dead (she never specifies why she thinks he's dead), she also says his eyes follow her. She runs away, not from the carnage, but from
him.
Dany's vision only encompasses a few sentences, so presumably everything she notices is important. Saying that this "must" be a vision of the Red Wedding requires us to twist what actually happened at the Red Wedding into a form that fits everything in the vision, and invent off-screen possibilities that allow the
entire vision to fit the Red Wedding, which is exactly where all the characters who try to interpret prophecies in this series get into trouble. We'd have to assume that Robb's body must have been stuck on Walder Frey's throne sometime after the slaughter (or that Robb was metaphorically presiding over the slaughter, which just isn't the case), we'd have to assume that the two crowns must be the same even though they're described very differently, we'd have to assume that the presence and placement of food can only be metaphorical (yet the dead king who's also still alive holds the lamb, not the purely "dead" feasters). We'd also have to assume that the lack of the RW's many defining characteristics---musicians, for example.---is irrelevant. I'm wary of making these kinds of assumptions about a vision, because that's exactly what GRRM seems to be telling us (via the example of Melisandre)
not to do.
Perhaps we'll get to the end of Book 7 and we'll never again have seen a group of feasters slaughtered in juxtaposition with a crowned Stark warg. But we have at least two books left, and I see no reason to assume that everything Dany saw in the hallway leading to the chamber of the Undying must already have been seen by Book 3.