I've done a little more digging.
Not really, you've just written a more elaborate summary than I did, for which I was far too lazy I mentioned the two versions as well as the channel layout in my original post already.
It seems clear that binaural Atmos playback is derived from the multichannel audio-atmos-2768 data. Neither the "audio-stereo-256-binaural" nor the "audio-stereo-256-downmix" referenced in the top-level m3u8 file are necessary for binaural playback, at least not on a Mac. They could be used on an iOS device.
But what did you use to playback the Atmos track on? Your Mac's internal speakers, external DAC, headphones? All that might matter of the selection of the tracks. Or it might not and the Music app on macOS always uses the full Atmos track? We simply don't know at this point. On iOS it's even more obscure at this point.
This casts further doubt, in my mind, on the idea that there are pre-rendered, device-specific files for playback on the internal speakers of supported devices.
Only partially. There are still references to the binaural and downmix versions, and we don't know yet when those are used. Possibly those are only for iOS devices. Surely disk space is cheap, but I doubt Apple would keep those version on a server if they were not relevant for something.