one way to handle things like this is to build in the triggers and criteria into the game itself. take the subjective opinion and honor system out of it.
If 70% triggers domination then as soon as that is hit, the feature to add players is removed. If that is too extreme, then you can only add players if an existing member has vacated a spot (on their own or by barbing out). If you shouldn't be able to barb your way to victory the game should literally not allow noble attacks on barbs at that point.
Likewise, inactives should be automatically detected, automatically given notice, and automatically deleted. Maybe there is a 10-day window that allows the current tribe to internalize. The rule specifics are irrelevant, at least they are known. There is no question.
Just as tribe leaders can set requirements to apply to a tribe, they should also be allowed to set stricter rules for their own tribe (like if you go yellow for more than 2 days you are gone, or if your points/bash haven't increased in X days, you're gone, or flagged or given a warning or whatever). If your barb/conquest ratio increases above a certain point, you should also be flagged. These are reporting tools for the council and leadership to better manage their tribes.
One of the biggest issues in the game also involves barb policies. That should be able to be set at the tribe level. The game can easily monitor it. If someone violates it, leadership is notified. Similar to a terms of service for any website, you can also have all tribe members agree to the terms of the tribe upon joining. The leadership should be able to see who has agreed and who hasn't. You can even have barb policies change at various stages of the game or domination thresholds met, etc. Or it could be time-based - like after the first 4 weeks.
This should be used in diplomacy as well with all sides agreeing to the terms (not just in spirit, but OFFICIALLY, as a function of the game). Example, if there is an agreement between two tribes were you have to ask permission before entering a province you aren't in, set that rule in the game. Make them ask permission (not in a PM but as an in-game feature). An alert comes up to the other members of the tribe (or NAP or Alliance member) that they have to agree to. If they don't respond within a given amount of time then permission is granted.
The same goes for an example of two or more tribes agreeing to their own contest. Let them set the rules. All parties have trust that they are adhered to because the game itself wont allow them to break it. Maybe it is the use (or limit) on crowns. Maybe it is a limit on players. Maybe it is a limit on tribe levels or NAPs or Alliances. For someone to break it, they would have to actively acknowledge in the game that they are bowing out and forfeiting the contest.
All of this is basic criteria-based rules that could be built right into the game itself. It actually isn't hard to do. There is already the permission functions within the game which is how you assign members access to various features within your tribe. Just expand it and most of the ambiguity, mod "discretion", etc is eliminated.