I don't think any of the other Movies Anywhere apps support sharing/playing the exact same downloaded file, but cross-app downloads are supported (buy on iTunes and download on Xbox Movies or Vudu or whatever).
The DRM does not require being online to verify once it's been set up
Incorrect, Apple did not build the software for all of the listed devices. Apple licenses the protocols and provides reference implementations through their MFi program.
https://help.moviesanywhere.com/hc/en-us/articles/1150045768...
Blu ray players are also locked down and the embedded keys are subject to revocation
In both scenarios the playback support is that the file just gives the credentials for the device with the Apple TV app or Airplay 2 to go and stream it from Apple's servers. No transfer of the video from a local download occurs.
You absolutely can.
A little known service in the US is “Movies Anywhere”. Four of the major studios participate in as well as does Apple, Amazon, Google, Vudu and other platforms. You link all of your accounts and a movie bought on one, automatically is credited to the other accounts as a purchase.
https://help.moviesanywhere.com/hc/en-us/articles/1150045768...