Definitely the diplomacy. As much as the warhawks in Rhomania may want the Accord to be crushed for their impudence, it is plain that the Ottomans are priority #1, and provoking this intervention into a full-scale war will not only mean trashing Italy (ironically making it both more difficult and more worthless to keep), but also likely force Sicily into, or be close to, the Accord and provide the Ottomans with the golden opportunity to try and break into Anatolia, with Roman attention divided again. Given that this is Ibrahim we're talking about, Demetrios won't want to take any chances.
Moving on, Genoa's independence and neutrality is guaranteed, but that does not preclude the rest of reconstituted Lombardy from potentially joining or be strong-armed into the Accord.
With four good medium-to-large economic and demographic players, the Accord could potentially be a fourth great player in European politics, with Germany, Triunia and Rhomania.