So after giving this a thorough workout again using all the accumulated wisdom to mess with various settings to no avail:
- Steam Playstation Controller Support
- Disable Steam Input for BG3
- macOS Accessibility settings for BG3 and Steam launcher
I've found the most reliable fix to be enshrined in this Reddit post reply
https://www.reddit.com/r/BaldursGat...;utm_term=1&utm_content=share_button...namely to delete / move / rename as appropriate, a plist file in the game related to controller definitions:
~/Library/Application\ Support/Steam/steamapps/common/Baldurs\ Gate\ 3/Baldur\'s\ Gate\ 3.app/Contents/Resources/GamePadDefinitions.plist
I have no idea what decisions the game code makes in the file's absence, and I'm not one for normally deleting 'random' files in the hope that something works (40 year IT career tells me there should be a logical reason!), but in this case it seems to be the most stable solution.
Over to Larian to figure out what interaction their code & this file is having with how macOS / Steam is presenting a single controller as potentially multiple controllers, thus fooling the game into the false multiple player scenario.
It's possible that the file will reappear at the next patch / hot fix, so one to watch. Hope this is useful for anyone else.