As for not being able to stop feedback from another account, I think it is a (quite logical) feature of librespot.
I can see, that it is a matter of security and separation of users. Anyhow, it seems to me a necessity from a usability point of view to allow at least local kodi users to stop playback (via a regular "stop" button).
We had situations like this: I start music via librespot, an hour later I leave home to fetch groceries. At this point my wife and children were forced to either listen to my selected music until the playlist is done or I return, switch off the AMP, restart the libreelec-box or call me so I can stop it remotely.