What?

If we accept these semantics (a collection of clear thinkers is a "singleton" because you can imagine drawing a circle around them and labelling them a system), then there's no requirement for the thinkers to be clear, or to communicate cheaply. We are a singleton already.

Then the word singleton is useless.

No-one generally has to attempt to agree with other agents about ethics, they only have to take actions that take into account the conditional behaviors of others.

This is playing with semantics to sidestep real issues. No one "has to" attempt to agree with other agents, in the same sense that no one "has to" achieve their goals, or avoid pain, or live.

You're defining away everything of importance. All that's left is a universe of agents whose actions and conflicts are dismissed as just a part of computation of the great Singleton within us all. Om.