[X] Unleash the Martyrs!
Between the two most voted options, I think this is the best one. Freddy is, in the end, the commander of our forces, and right now he doesn't even know the status of most of them. If/when the enemy break through the gates, then he can rally our troops and come to the defenders' help, but in the meantime he will be a lot more useful by putting his tactical expertise to good use, and the walls are the best position from where to do so. Let Freddy recover a bit for when Tullaris or some other enemy elite inevitably tries to directly fight him, but it doesn't look like they're immediately going to take the field, so for now Frederick is IMO much more useful as a leader and tactician.
I'm also not sure about the fire support variant, BTW. The artillery firing just before the flagellants charge might be counter-productive, as if the Druchii disperse themselves a bit to minimize damage that might reduce the efectiveness of the bomb-wielding flagellants, who will cause more casualties the thighter the enemy soldiers are packed up.