Voting is weighted, so 2/3 of the weighted votes. (I only referred to unweighted votes as the most democratic tie breaker.)

On motion 5.5, wouldn't enslavement of Gauls be almost as good for your project as extermination? It still adds to our coffers, looks like raiding and also adds to our existing settlements (you might want to make sure we have generals in the settlements you particularly want to benefit from the influx of slaves - personally, I'd recommend Capua and Ancona, as well as Roma, of course). Enslavement might be better than extermination if the intention is to let Gaul to survive and/or to repeat the exercise later. IIRC, we don't have a houserule against demolishing structures, so you are already free to do that.

But if FLYdude or shifty157 were elected with a motion not to invade Carthage, they would have to follow the motion or face the wrath of the Senate. In the latter contingency, I'd certainly vote for them to be removed from office in the mid-term (2/3 majority required). Of course, we don't have such a motion tabled, so that case is purely hypothetical (voting against a motion mandating an invasion of Carthage is not the same as voting for a motion not to inavde Carthage).

We don't really have a contingency for an emergency referendum at the moment, but there is always the mid-term session if you absolutely have to exterminate.