“That’s a good point, but I think you’re behind on some of the context on what we were discussing. Can you try to get more of a feel for the conversation before joining it?”
It gives the person an understandable reason for their misstep. (“Sorry, I must have misunderstood what you were talking about.”)
It gives the person a reason to stick around. (“I messed up. If I want to correct this, I need to listen and get more context.”)
It adjusts the person’s behavior in future interactions. (“I should get a feel for the conversation before joining it to avoid messing up in the future.”)
It’s no more aggressive than it needs to be to allow you to disregard what the person said and continue your conversation.
What little aggression is there is reasoned and grounded in something easily understood, so it doesn’t come off as rude.
The above sounds better in text than it does in an actual conversation, but the same principles should apply in an actual conversation. “Name, hold on. I think you’re missing some context. Can you listen for a few minutes to catch up?”
I think your focus on payoffs is diluting your point. In all of your scenarios, the thing enabling a defection is the inability to view another player’s strategy before committing to a strategy. Perhaps you can simplify your definition to the following:
“A defect is when someone (or some sub-coalition) benefits from violating their expected coalition strategy.”
You can define a function that assigns a strategy to every possible coalition. Given an expected coalition strategy C, if the payoff for any sub-coalition strategy SC is greater than their payoff in C, then the sub-coalition SC is incentivized to defect. (Whether that means SC joins a different coalition or forms their own is irrelevant.)
This makes a few things clear that are hidden in your formalization. Specifically:
The main difference between this framing and the framing for Nash Equilibrium is the notion of an expected coalition strategy. Where there is an expected coalition strategy, one should aim to follow a “defection-proof” strategy. Where there is no expected coalition strategy, one should aim to follow a Nash Equilibrium strategy.
Your Proposition 3 is false. You would need a variant that takes coalitions into account.
I believe all of your other theorems and propositions follow from the definition as well.
This has other benefits as well.
It factors the payoff table into two tables that are easier to understand: coalition selection and coalition strategy selection.
It’s better-aligned with intuition. Defection in the colloquial sense is when someone deserts “their” group (i.e., joins a new coalition in violation of the expectation). Coalition selection encodes that notion cleanly. The payoff tables for coalitions cleanly encodes the more generalized notion of “rational action” in scenarios where such defection is possible.