I'm thinking the WC could work more like the Majatran Union used to - a forum for the various nations to collectively debate stuff, and also pass officially binding resolutions, but whose power depends entirely on the will of the various nations to enforce them. To give a RL comparison, it would probably be more like the League of Nations than the UN - founded on idealistic peace-preserving principles, but unlikely to get anything done unless all the big players agree. For that, I think the model proposed by Zanz would work best - resolutions might get passed by the General Assembly or whatever, but they won't be realistically enforced unless a significant number of nations ratify them via treaty. I also think we should have a Security Council composed of all the Great Powers as defined by the RP team's military rankings, with each being given the power of veto.
As for the ITB, I could see it become of the WC's institutions, but with its decisions also dependent on the will of major actors to enforce them. You could have, for instance, some highly developed nations pressure the lower ones to change legislation in line with ITB recommendations, while the economically weaker nations would have the power to outright reject them (and maybe be supported by other highly developed nations in that decision).
I also think that when the WC is founded it should automatically include all nations, and then it could decide whether it would accept or reject secessions or mergers or governments in exile. Having it based on a particular treaty would be unworkable, since there's no guarantee everyone will ratify it and then remain in it.