Hi,
the json export file is not ment to be edited by users to be imported again, because that would circumvent some security features, e.g. users importing Javascript code, or modules with wrong ids that potentially could mess up the database or 1000 modules to manipulate the Popular Modules statistics.
If you change the JSON file, the import will fail.

Otherwise, if you're adding modules to a rack, how do you tell it to add to the next row? For example Ihave my top row done, but there's still space.

Users can move modules around with keyboard shortcuts like enter key and cursor keys but that is done via Javascript and I think there are no web accessibillity features for that currently.
There is also a function Optimize Rack Space in the edit menu that will try to fill out all spots but will shuffle all modules inside the rack around.