That’s pretty awesome, thanks a lot for sharing! 
dbus has a lot of of untapped potential, I think… including general lack of good docs too. That makes it possible to find interesting solutions like this by poking around the applications connected to it.
Your solution is well thought out, maybe the one thing would be to make sure that at least some interface is connected to outside. If I understand this correctly, if this code runs before the main wifi interface gets a chance to connect, it will be “unmanaged” too? (e.g. if there’s a slow access points). Would be good to avoid that sort of race condition. Maybe it would work even better with the persistent interface names even in this connman case, as you’d then always know what interface to unmanage…
Now I got to dig in the code a bit more 