Apologies for not yet supplying J-E with a most detailed account of where I found my things broke down.
What I found was that the default naming of MacOS’ in-built pre-supplied virtual driver which is identified as “IAC Driver” within MacOS, wouldn’t work with O.Stage.C. until I renamed it in the actual Audio Midi Setup (Applications -> Utilities -> Audio Midi Setup). I renamed to simply “IAC” (in Audio Midi Setup) and after that, following the also correct way of Id declaring in O.Stage.C. then communication works.
The fact that O.Stage.C. allows us to declare an Id of our choice just with no spaces and thus no need for quotes does not affect this version of the problem. That guidance should be followed anyway and on top of (IE still make sure your Id naming in the server config follows the documentation guidance!) - so as well as having that understanding of using an Id in O.Stage.C. mac users also need at the system level to not have their devices be named with spaces in them.
I believe it’s entirely straightforward to replicate the error on MacOS by trying a configuration where the system’s name for the midi device has a space, vs when it doesn’t. If there are still problems once the device is renamed at system level, I would suggest there are other problems, and that this is still required. I haven’t proved this by way of checking additional systems, this is my anecdotal experience though.
(Once again J-E thanks for your immense project/gift! Without this culture and dedication we are all
so much poorer)