roku has specific requirements for it's json file, that is supposed to be "industry standard", however, upon researching, I can find no definitive standardfor the syntax used in the json file pertaining to video files.. It's very likely that jw players format is different from rokus.. I would suggest starting with one video file and getting it to ingest into the direct player before trying it with a complete library of files.. Then you'll be able to find out whats what..
Also the direct player has some limitations, 15 categories and 40 videos per category. I ran into that undocumented roadblock immediately and have to create several different channels to get all my content on roku..