I feel inclined to point out that, unless I misunderstood things, the interpretation that Thorek wants is the one that happens to align with the apparent reality. That is to say, it's the truth.
What's our position on useful, dangerous information?
Thorek is basically asking us for a bloody shirt to wave. In this situation, the shirt is definitely bloody, but because of our lack of information, we can't be certain what exactly caused the bleeding. Having support from the rest of the Runesmith Guild may very well have done nothing for Karag Dum; without Karak Vlag, they were simply too isolated within the Chaos Wastes to receive meaningful support. They could have fallen anyway, especially since Dum was cut off by a full-blown Storm of Chaos. Isolation from the Guild most certainly didn't help Karag Dum, but we can't prove that it was sufficient on its own merits to cause the fall of the Hold. It's supposition, but as the leader of the Expedition, we get to write history to some extent. Whatever we say will be given great weight.
I think regardless of what we say, Thorek
will use this as an excuse to begin taking pot shots at the rest of the Runesmiths. It would be more equivocal without the support of the official report, but there's still an underlying truth that the Runesmiths abandoned Karag Dum. Attributing culpability to the Runesmith Guild in the official histography merely accelerates the timeline Thorek's probably already put together in his mind and minimizes the amount of bullshit and equivocation his political opponents will make. He has made absolute no bones about what his goals are: he's going to break the guild and reforge it into something better. He's clearly driven and as a dawi the fact it could take decades or centuries is irrelevant. Refusing to assist Thorek in this endeavour doesn't make the problem go away, it merely means his reckoning will be delayed to some other time.
In fact, I think regardless of what we do, Thorek is going to move as quickly and as vigorously as he can on this issue. If he doesn't then it likely means the more reactionary elements of the Guild will move to censure the rune pokers of Karak Vlag. While Vlag is almost certainly
not what Thorek has in mind when he discusses wanting to more freely share Runesmithing secrets, they're likely to be much closer than the ultra conservatives that currently dominate the Guild. If Vlag is successfully censured, then it makes Thorek obtaining his goals that much harder; there would be solid precedent against the direction he wants to move. If Thorek can point to Karag Dum as a mistake, with the official report blaming the Runesmiths for isolating the Guild of Karag Dum, it makes it much harder politically for the Guild to do similar things to Karak Vlag which in turn become precedent for a more tolerant and permissive Runesmith Guild.
I think the key take way point is this: Thorek is looking to pick a fight. This fight will happen and with Karak Vlag returning with the Rune Pokers, there's already pre-existing flashpoints. A long, drawn out disagreement between Runesmiths is probably the worst outcome as that provides the greatest opportunity for people to become invested in the argument which then increases the likelihood of Grudges or Runesmiths going Slayer if they lose out politically. Thus, shouldn't it be in our best interests to weight things towards Thorek as much as possible? We do want his viewpoint to win
and by giving him advantages, we shorten the overall length of the fight that will happen.