I'm not sure if this will solve your problem, but it may at least give you a hint of where the problem is.
I compared the converted JSON files compared to yours, and noticed that the JSON file you are using does not indicate which texture the object should use.
Add
"mapDiffuse" : "nameoftexture.png",
to
"materials: [{ ..., ..., ... }]"
an array.
Good luck.
EDIT
Your model seems to work with textures for me when I added this line to the material properties array.
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