Just stick with the bit about how there is a seemingly impossible diplomatic breach to address, find a way to address it, keep some extremists (like Tarka?) on one or both sides so the show still hits its season finale explosion quota, and end with some inspiring monologue.
It's Star Trek, not rocket science.
This is just me keeping steady with my theory from the beginning of the season that it is far less about who, and far more about the themes of miscommunication that were smacking left and right as early as the season premiere. If I'm wrong, and this ends up being the work of a genuinely unapproachable threat of mass evil, well, I'll be surprised.
But you don't put Book in the "seemingly justifiable vengeance" hotseat in a Star Trek show without some degree of subversion in the works!