GMs choosing what strategies work sounds pretty iffy. Sounds more and more like putting the answer before the question.
I feel like mock UN would work better as a rhetorical exercise with a single specific issue, where different regions have a sheet of objectives to accomplish and see how well the various groups work together to come to an agreement. Flex your debate skills pretty hard when you get Saudi Arabia and have to defend your right to execute apostates or something
Limited scope: specific objectives is the way to go imo. It's pretty important as well to make sure to ground those objectives in reality/history and make sure not everyone can win. The 'ideal' solution for Israel/Hamas might be for both to drop their weapons and hug it out, but that doesn't mean that will be happening anytime soon. MUN players are usually disconnected from their nations in the sense that they are either lacking the historical perspective, distrust, and are not-actually-dictators so they are too willing to cooperate with eachother, or, they don't get in character enough, and exploit that to push around other nations with uncharacteristic aggression. You also have the problem that the simulation begins and ends at a set time. If "maintain good reputation" isn't on your objective sheet then your best bet is to backstab everyone who you should be most inclined to agree with, not taking into account the long-term results.
We once did a pseudo-MUN thing in my highschool where we simulated the Treaty of Versailles. While maybe not entirely accurate, it was pretty successful mostly because
A: At most ~75% of the leaders could get 100% on the project, tied to their objective fulfillment
B: There was no option for unconditional cooperation, and your objectives often had you deliberately fighting other players. For example France wanted to grind Germany's face into the dirt, but Britain saw that as a precusor to another war. Italy wanted to acquire land, America was convinced that was the eternal well of european conflict.