Nope. Only that it’s highly unlikely to ever arrive in Cubase 12.
Currently, that would be a “no”. You’re the second one to request per-device visibility settings, and I’m generally interested to make this possible one day, but it is not my priority right now. Also, this might be of limited use at the moment, since visibility agents have no effect at all without setFollowVisibility() working.
I don’t think so, sorry.
That’s a very interesting idea and I’d love to have this. Still nothing I can do on my own without the API providing at least basic support for it 
One of the first things I tried when I realized setFollowVisibility() had no effect was hiding tracks in the project window. Sadly, that has no effect either, so I doubt visibility agents will make a difference.
Perfect candidates for the “Track” encoder assignment. I’ve been toying with the idea of adding them there, but I was on the fence about whether they should be there by default or hidden behind a config flag. Now I think there’s nothing wrong with just adding them to the list of “Track” assignments.