Is MRN data in JSON format?
Yes, the content data is in UTF-8 JSON format.
The content stores in field id 32641 "FRAGMENT" which is a buffer type compressed in zlib algorithm. You have to determine whether a particular NEWS item is single fragment or multi-fragment. If it is a single fragment, the applciation can decompress the buffer to JSON content right away. For the multi-fragment, you have to assembly all fragments's buffer together, then decompress to the JSON content.
For Elektron API tutorial, please see this link