Edit : @Alratan, I like your take.
Thank you. The way I see it, there's something up with Thorek's timeline, whether or not the details I posted about the foundation of Grungni's Tower before Middenheim are accurate.
For the dwarves to go to settle amongst humans there must have been settled humans. Humans in the northern Old World didn't settle down from at least seasonal nomadism until around fifteen hundred years after the Time of Woe began. Around the time or as they did so the Empire was founded by Sigmar and uplifted by the Karaz Ankor, who we're going around teaching them to work iron and steel and build in cities and roads in stone, and write, and such like.
Firstly, what was Karaz Ghumzul doing for those fifteen centuries. They were the neighbours of Laurelorn, for somewhat under five hundred years they were neighbours of the Belthani, and for something less than another five hundred years they were neighbours of the northern proto-Imperial tribes. At least initially they were apparently spared the battles with the elves, and during the Time of Woe would have been spared the worst of the skaven (see below), and would have been hit later and less hard by the greenskin migration from the east, simply because of the geography. What may they have been up to during that period, both internally and with those neighbours during that stretch of time?
Another consideration, according to the maps we have the skaven didn't have tunnels stretching to the Middle Mountains until after 1000IC, so would have had to go overland to reach them, which would have severely limited the forces they could deploy.
So, conspiracy theory time. Let's say Karaz Ghumzul fell during the Imperial era, as Thorek rather suggests. This was the period of the early Empire known as the Drive to the Frontiers, when they tried to conquer, expel, or eliminate just about everyone in their path. Human tribes such as the Fennones were conquered or driven out like the Norsii, the greenskins and beastmen eliminated as their forests were felled and the elves of Laurelorn were fought with but survived when their forests weren't.
If Karaz Ghumzul still existed during their period it would have probably still been considered a renegade, traitorous province by the Karaz Ankor, and so possibly fair game to their allies in the Emperor of Man. Is it possible that it was
the Empire that defeated Ghumzul, and handed the defeated population back to the Karaz Ankor, possibly while the secondary settlements under Middenheim switched sides/surrendered early on realising their position was untenable?
That's something that could have easily been erased from history as a shameful episode for both the Empire and the Karaz Ankor.
After all, who else could have defeated Karaz Ghumzul if it survived into the era of the Empire triumphant. The skaven couldn't reach them without marching overland through the Empire, and the Empire had the beastmen and greenskins beaten…
In the unlikely event this is true, Thorek would quite possibly be unaware of it, and we could be stumbling into a political minefield, or remains of a minefield, just as bad as the Dum debacle.
Also, if Karaz Ghumzul is a Golden Age hold, is it a potential Waystone on the dwarven network? Depending on how old the dwarven settlement of Grungni's Tower is, is Middenheim? Even if not, it would be an obvious candidate to make into a regular Waystone, so it might be worth a look. The Cult of Ulric and their Eternal Flame might be connected to it if so.