If waystones to the north of you prevent the winds from traveling south, then you wouldn't need waystones in the south. Just build a giant wall of waystones up north and then shrug and forget about it. And the south too, the winds also come from the southern polar gate. You do need moving waystones to avoid your waystones turning into dhar bombs and damning the entire region to recreate the chaos wastes in miniature because the nexus they are linked to got wiped out by a warpstone meteor.
Draw a circle with a radius of half the distance between Eicheschatten and Mordheim.
Here's a map to do it on. Is everything in that similar to the Drakwald and Sylvania? It's been five centuries since then. More than enough time for the waystones that linked to Mordheim to accumulate magic and then be blown open by even the gentlest Storm of Magic.
The Hedgewise claim to have been charged with the upkeep of waystones by Sigmar. They even held two nexuses at one point: the Brass Keep and the Marcher Fortress.
You even suggested earlier that they along with the Belthani pruned the vines of the waystone network. Still, this was the Era of Three Emperors. They would not have had the capacity to manage the hundreds of waystones. No one would have had that capacity. Not the Ostermark and Stirland Hedgewise. Not the pathetic last remains of the druids in the east. And do not doubt, it would be hundreds.
And Waystones to the north don't do a perfect job, but they clearly intercept some Winds. That's why places with no Waystones like Sylvania aren't actually part of the Chaos Wastes and daemons don't cavort through streets where the buildings cry tears of blood from the screaming faces melded into the stone.
I don't think we know enough about the network topography to say what the determinants of what Waystones were connected to which Nexus were. It may not just have been distance. For example, for all we know there was a general preference for magic in the Empire to flow from east to west, so you wouldn't draw a circle around Mordheim, as lines of Waystones west of it would tend to
Or it could be based on elven settlement patterns, and if the nexus at the city that became Mordheim was settled relatively late then Waystones that were closer to it than to other nexuses closer to the coast would already have connected up to them.
We just don't know, so can't really draw strong conclusions from it about how many Waystones were connected to it and where they were.
The local Hedgewise and Belthani may have been able to do something or not, but we don't know what their capabilities were or weren't, so we can't say whether they could have done something to those unknown number of cut off stones.
If hundreds of Waystones in the eastern Empire, including her native province were dhar timebombs just waiting to explode then I think that Mathilde would have mentioned this when she learned that Mordheim had been a nexus.
It's a slightly different topic, but while the Hedgewise claim several things on the basis that it's knowledge that has been (orally?) passed down without deliberate invention or error by a persecuted group for over two thousand years. They've no evidence for this, and even if they completely believe it today, they're also very convenient and flattering thing for a persecuted group to tell themselves. It's similar to how the various Hedgewise groups have a set of different foundation myths for the source of their abilities, crediting various Gods or their historical identity as priest-kings. Forgive me if it all sounds a bit like ancient Masonic secrets passed down the generations from the builders of the pyramids to the Freemasons today.
If some Hedgewise did know some Waystone access codes, they could easily have learned them later from forest dwelling Belthani they met wile living on the boundaries between civilization and those forests.