When I export a map as a json, how do I input it back in to make more modifications?

You can't import it back in the generator, only in the City Viewer.

That would have been my question too. Once I have finished a map – or thought it was finished – is there no way to upload the map saved as json, png or svg back into the generator to edit it again?

Unfortunately, the permalink isn't helpful either, because it doesn't ‘save’ all the details (such as changed names for districts) ...