But if the neck has been broken badly enough to cause death, that means that there has been signficant disruption of the nerve pathways from the instance of the break.
You simply cannot break a neck specifically enough that it paralyzes respiration but leaves the individual fully mobile as Randall's zombie was.
At very best, you'd have zombie that could paint a picture with its left foot....
I did say it was an unsatisfactory answer . . . right?
The whole zombie thing really doesn't make any sense from the get-go anyway. How do you "re-animate" a dead body by just activating the brain? The whole body is a metabolizing machine. You breath and take in oxygen and exhale carbon dioxide, eat to obtain "fuel." Zombies apparantly don't breath as they can walk around underwater (according to some zombie fiction atleast). Some need "brains." The walkers apparantly muscle tissue.
But all the time their bodies are decaying.
So, on one level, worrying about broken necks seems a bit superfluous. It doesn't make sense a zombie with a neck as broken as you suggest should be able to move .... but then, the whole zombie thing just should not work.
But then, there'd be no AMC series and where's the fun in
that? ? ? ?
Oh, and let's not even talk about the fact that when they sink a brush hook or clever into the front of a zombie's skull they're not coming anywhere remotely close to disrupting the brain stem.
They are, essentially, hacking dead tissue and the zombie should still be fully capable of running down some human tartare...
I think you're over-analyzing the problem.
Crunching up even "dead" brain tissue might not sit well in zombiedom.