I have the same error messages on my Pi Zero W running Balena Sound as the OP does in his 3rd and 4th images.
Two every second, one says “23.05.20 12:45:36 (+0100) snapcast-client 2020-05-23 11-45-36 [Error] (worker) Exception in Controller::worker(): connect: Connection refused” and the other “23.05.20 12:45:36 (+0100) snapcast-client 2020-05-23 11-45-36 [Error] (stop) Error in socket shutdown: Transport endpoint is not connected”.
However although this doesn’t seem to prevent me from playing over bluetooth, it won’t play as part of the multi room set-up with snapcast and it also seems to mess with the Spotify connection. Sometimes Spotify is listed as running on the device but it does not appear in my spotify devices list and sometimes it just quits.
It has worked sometimes and then it stops with no obvious change. It spends much more time not working than working though.
I have a justboom digi zero on the raspberry pi and have provided my spotify credentialsas fleet device variables.