Most MP4 muxers will not be able to preserve the JOC metadata and/or the proper codec signaling to a downstream decoder and you will just be left with 5.1.
If you need to demux/remux an mp4 with DD+JOC you will need to use the open source tools that Dolby has posted on Github. Links to them and their usage is described in this article:
However, if this is Dolby Vision test/marketing material I discourage you from remuxing this with SDR as this will create misleading material. If your platform is unable to decode HEVC I suggest that you contact the Dolby representative you obtained this from and ask if a h.264 version is available and/or there is alternative material that does not reference Dolby Vision.
Spatial coding is always part of encoding to Dolby Digital Plus JOC. It is not used at all in Binaural Rendering. The number of elements used in Spatial Coding to Dolby Digital Plus JOC is dependent on the datarate. Encoding at 384kbps uses 12 elements, encoding at 448kbps and higher uses 16 elements. 16 elements provide sufficient resolution for spatial coding even with a full track count of 128 beds and objects reproduced on speaker layouts up to 9.1.6 and higher. The efficiency of Spatial Coding works in part because even with high track counts often not all inputs are active at once. It is remarkably transparent but not entirely transparent with some content. That is why Spatial Coding emulation is available in the Renderer. You can A/B test out the results. Spatial Coding emulation should be left on during mixing to audition the effect of Spatial Coding and if needed, make any mix adjustments necessary to ensure you are satisfied with how it would sound encoded and streamed on OTT services or on Blu-Ray.
Hope this helps.