I agree that this is important. Are you more concerned about cyborgs than other human-in-the-loop systems? To me the whole point is figuring out how to make systems where the human remains fully in control (unlike, e.g. delegating to agents), and so answering this "how to say whether a person retains control" question seems critical to doing that successfully.

and increasing the number of actors can make collusive cooperation more difficult

An empirical counterargument to this is in the incentives human leaders face when overseeing people who might coordinate against them. When authoritarian leaders come into power they will actively purge members from their inner circles in order to keep them small. The larger the inner circle, the harder it becomes to prevent a rebellious individual from gathering the critical mass needed for a full blown coup. 

Source: The Dictator's Handbook by Bruce Bueno de Mesquita and Alastair Smith