It loads the tempoary cells that are defined in the Tamriel worldspace. AFAIK the temporary cells defined to the child worldspace with the same coordinates do not matter at this point. The temporary cells of the child worldspace are only loaded when entering the child worldspace, superseding the temporary cells of the Tamriel parent worldspace.
The persistent cells of all child worldspaces are always loaded together with the persistent cell of the parent worldspace. They seem to act like one and the same.
Crash fixes is not reporting a problem with a nif? Then maybe a deleted record or a navemesh problem. If the crash is that reproducible, you should be able to determine the coordinates of a handful of cells that attach when the CTD occurs. Find them in xEdit with CTRL+SHIFT+F cell finder, then have a look at plugins that change them. Remove those one by one. Once you found a plugin, you could have a look at the references and other records in those cells, or the overwrite cell record themselves. You get the idea.
Only if all that fails to find something would I consider looking at the cells in the childworldspace, however unlikely it seems. I would still check, because more often than not, these things are not always what one believes them to be...
So to extrapolate from what you are saying, presumably as I can get right to the cell boundaries in the Tamriel worldspace, if both the Tamriel and WindhelmWorld persistent cells load together they're not going to hold the error as they will have been loaded a long time before I get to within half a cells distance from the walls? But if it is also the case that the WindhelmWorld temporary cells only load if you are inside the city, and the Tamriel temporary cells only load if you are outside the city why does the crash persist both inside and outside the city?
It is 100% repeatable.
There is no Crash Fixes mesh warning, and one of the first things I did was to run everything in meshes/architecture/windhelm through NifHealer which reported no errors.
I have checked different times of day and different weathers to rule them out.
And as far as I can tell it is every cell containing Windhelm as per the area within the red lines in the image above. So I need to check through all of those cells presumably (both in Tamriel and WindhelmWorld?)?