You can use JSON to do the imports, but 1) if you don't export with "Nonessential" checked then the JSON loses some data, like orange mesh edges, and 2) you will need to use the same version you exported the JSON with (eg 4.1). If you use a newer version, it may not work. With a .spine file it will always work.
It's always better to have the .spine file. The JSON file is only for loading the skeleton data at runtime, it is not a good format to store the skeleton/project. If you only have the JSON file, import it into Spine and save a .spine file. You can also do that via the command line:
Spine -i skeleton.json -o skeleton.spine --import
I did the export 7 times with 4.2.23 and it was 888px wide every time. Please try the latest 4.2 (4.2.23).
