well i kept thinking about this a bit … and thought of the ‘watcher’ proposal and then i thought ‘what would happen if one randomly selected other group would check on a group on a regular basis and could nuke that group somehow if it would be bad (or have it re-checked if suspicious and nuked if the others agree)’
of course that would introduce additional complexity in the system (and obviously my calculation has an error…) but what i did was just adding the scenario with a re-check (just multiplying the initial probability of controling a group with the probability of controlling a randomly selected other group as well):
that would push the chances for a sucessfull conquering (realistically) to somewhere ~50% network share
though my calculations are wrong 
this indicates that a checkup by another group would largely increase stability agains bad actors conquering the whole thing (disruption still would be the same - but not sure if that would be a problem if a disrupted section just could get kicked out of the network [as long as nobody tries to just get half the network kicked out to overload the rest of the network with data…])
especially when doing the calculation with 100 000 000 elders instead of 100 000 elders a re-check would have a large influence
(on the other hand i probably should first get my numbers right before making other statements … that drop with the re-check in the end doesn’t make sense …)
[but it totally makes sense that if the network selects who checks on a group the probability of success suddenly is again highly dependent of the probability of being in control of one group … no matter how far off from the average probability the network happens to be locally]