I think that this is fine with a class of 15-20 people, which is too large for the in class session, but not so large that you will have inactive players (a problem in this game). Though if you assume that inactive players are countries following isolationist policies or with domestic troubles, they might have some interesting things to write in their papers. I also found interesting the commonalities and difference between the end states of the world among the different sessions.
Diplomacy at its core is a game of Offensive Realism. In most worlds created as you can see aggression was the norms, with 2 powers usually creating fairly large concentrations. Balancing is very hard to achieve, and essentially Mearsheimer is validated in his criticism of Waltz. That said some of the worlds saw little aggression, and this indicates that rather than the structure (rules of the game), the willingness of the players to care or be aggressive is also an important thing. Power politics might have to do with culture as opposed to necessity.
And interesting but time consuming exercise is to run multiple diplomacy sessions with the same players, but change the rules for each session to conform to a different theory of international politics (Offensive Realist, Defensive Realist, Liberal, Steps to War), and then have the students compare their behavior in different games to see if rules matter more than motives, or how rules and motives interact to produce behavior.
For the fun here are the different games produced by my students
Poland Triumphant , Ottoman resurgence. I am pretty sure those two cut a deal and stuck to it.
Egyptian Hegemony (Poland, Spain and Italy were also aggressive)
Anglo-German dominance. Pretty much the UK conquers the world.
A rather stable system with little aggression.