If you have someone who still hates you, have them educate your children (+40 for 2 of them) or grandchildren (or brothers/sisters), or plant your chancellor in his capital ahead of time. The tournament is good for +10 for ALL your vassals, and the feast is good for another +10 for everyone who shows up. Then, when you have that one moment of peace do whatever it takes to pass the law change. Or, if that happens gift a couple hundred gold to one side and hope that they hire enough mercenaries to win the war. Don't answer allies' calls for war, don't go to war yourself, and pray that you don't have one of those decade-long vassal wars where one side has 265 men and can't siege because there's 500 men in the castle.
Save your Grand Tournament, save your feasts, and save a LOT of gold to bribe unhappy vassals, and then just keep a close eye on your realm.
As you've probably noted, since vassals fighting each other is probably your biggest concern, get everything ready for that one moment.
