I think you need to consider that thread is from 2015. That said, the linked post from Jodal contains info that is still pretty relevent. Except init.d is long gone (or as good as) and I’m not interested in looking at that. You need to be editing the systemd service files for this to work, not init.d scripts.
I think you’ll find more useful content in the far more recent thread at Multiple Mopidy Instances/SoundCards | One Server - #8 by Cephas1029