I'm not familiar with Warhammer canon but the Karaz Ankor timeline has always stumped me on one major issue:
- The Chaos Dwarves believed their Ancestor-Gods abandoned them, ergo, Valaya and her siblings existed & performed their deeds before the Split happened.
- The Dwarven Ancestor-Gods were known for overcoming the Coming of Chaos (AKA Polar Gate breach), thus, the Norse Dwarves split from the Karaz Ankor after Runes were invented, after Caledor Dragontamer created the Vortex.
- Dwarves expanded North through the World's Edge Mountains before finally splitting off to Norsca/East. Johann witnessed Grimnir mining Gromril in K8P, which is a Southern Hold. Ergo, K8P was established before the Norse Dwarves split off.
So I'm struggling to fit these facts coherently since it doesn't make sense for the Norse Dwarves to have split off before the initial Coming of Chaos.
This is mostly theorycrafting on my end (and Rhunrikki Strollar), but i think that the struggle is that you are mistaken about the oldest hold.
The dwarfs originated in the southlands, not at Karaz-a-Karak (The oldest recorded Hold is Karak Zorn), and migrated north, following the mineral rich worlds edge mountains.
It is likely that this was the closest stronghold to the original homelands of the Dwarfs in the southern part of the Worlds Edge Mountains
Karaz-a-Karak and Eight peaks were probably founded at the same time period.
The norse holds and the eastern holds, were presumably founded later, still before the great incursion, but well after K8P and KaK, and they were never connected to the underway, because the incursion happened.
Uzkulak was probably a step towards the mountains of Mourn, where the modern day ogre kingdoms are. According to Snorri, the dwarfs did reach there.
When he great incursion happened:
-The north becomes so demon infested, that the more southernly holds believe "nothing could have survived" and give up trying to make contact. It was so demon infested that Snorri Whitebeard and Malekith said "fuck that". The Northern holds were under siege for longer, for a significant part of the golden age, and didn't venture south after that ordeal was over. There wasn't a split, as much as a separation by time and distance.
-The eastern dwarfs of Uzkulak fall to Hashut, and migrate south, creating Zharr Naggrund, a complete departure from the dwarf cultural norms: It's a fortress-temple-city built on a massive plain, as opposed to a fortress-city delved into a mountain. For their blasphemy, and probably worse sins, Karaz Ankor declares an eternal grudge against Chaos dwarfs. There were probably several wars of extermination, but even with golden age level power, they never destroyed Zharr Naggrund.
But how come Barrak Varr didn't hear anything about them from Human traders or explorers?
I have no idea. Warhammer canon is a mess?