I feel like there is some overthinking of the monty problem here. The answer no makes the most sense. Consider this, what is behind the doors is predetermined, right? So either the smallest number of people you can hit is 0 or 5. The most is 10. Chosing either a or c guarentees at least 5 people hit. Choosing B limits the number who can be hit to 5 (if they're behind the door). So you should always choose B and always keep the choice.
Another way to look at it is: How would you solve this problem with email?
The reality is, there is no way to solve the problem of moderation across disparate servers without some unified point of contact. With any form of federation, your options are:
The reality is, any federated system is gonna have these issues, and as long as the protocol is open, anyone can implement any instance on top of it they want. It would be wonderful to solve this issue "properly", but it's like dealing with encryption. You can't force bad people to play by the rules, and any attempt to do so breaks the fundamental purpose of these systems.