Similarly to the rationale given in the historic thread for your other snap (Human Review Required for audio-playback and audio-record plugin cluemaster-mediadisplay-core) - +1 from me for granting the ability to slot audio-playback for cluemaster-videoplayer-core as you are shipping your own pulse-server and managing these Ubuntu Core devices yourselves.
Regarding alsa and hardware-observe this also makes sense given this use case, so +1 from me for auto-connect of alsa and hardware-observe for the purposes of audio playback.
Finally, regarding shutdown - as you intend to control these Ubuntu Core devices this also seems reasonable, so +1 from me for auto-connect of shutdown.
One thing I should ask however, have you considered a dedicated snap store https://ubuntu.com/core/docs/dedicated-snap-stores for your devices? This would allow you to control what gets auto-connected etc, particularly since you said this snap is not intended for public installation.