So i did some more tests and still could not recreate your issue.
So when you click “install” in the Bluetooth tab, the Spotify connect player stops or is no longer playing music? It would be very helpful if you could send me the debug log.
About setting up the hifiberry: As i mentioned, there are problems between Squeezelite and Spotify Connect, so when you choose “Simple Airplay” the Squeezelite player is not running. When choosing “starter” or “advanced” it is running.
For a better understanding: you choose “simple airplay”, start squeezelite, install Squeezebox and Spotify and everything runs?