I'm trying to do some research on the Middle Mountians but the Wiki a fricken mess and I think GW has been wonky with their record keeping again.
For a start, it appears that the Middle Mountains was the location of a third chaos gate, which Grimnar spent many centuries fighting until his heir could replace him. I can't find a real source for that because citations are for nerds apparently, but I'm assuming it's End Times rubbish because it feels like the sort of pointless escalation that End Times would introduce. "Oh ho, you thought there were only two chaos gates? Well, you're wrong, there's a third, right in the middle of the most important nation in the story!" It also appears to contradict the story of Grimnir dying in the Chaos Wastes, but there's not enough information for me to say for sure.
Next up, there are two abandoned dwaven Karaks in the Middle Mountains, and both "were sealed with curses" after being abandoned. There's Karaz Ghumzul, from Sigmar's Heirs (WFRP 2nd), which fell during the Time of Woes (4000 years ago), and Karak Kazarak, from City of the White Wolf (WFRP 4th), which fell 200 years before Sigmar (2700 years ago) and the survivors helped the Teutogen tribesmen build Middenheim.
Kazarak is described as the "only dwarfhold in Middle Mountians during the time of the primeval Empire", whilst Ghumzul is a "short lived dwarf realm from the golden age". Additionally, Kazarak is in the western foothills, whilst Ghumzul is somewhere south of Wulfenburg in the east. But the fact that both holds were cursed after they fell, and the populations merged with local human populations implies to me that they are supposed to be the same place, only with names and dates changed between editions.
And if they are not the same place, and both exist in DL canon, then that means there are two communities of dwarfs in Middenheim from different dwarfholds. I can't imagine the Kazarak refugees would have been happy merging with the Ghumzul traitors.