[johnsjmc]

OK Terminology. All wiring downstream of a breaker is a branch circuit. I think I understand your setup is all branch circuit but you are differentiating the underground feeder as a supply and the above ground wire in the buildings as branches. I would have set up a subpanel in each building or at least a seperate branch circuit for each not a single breaker serving more than one building (which is illegal here also) and then trouble shooting would be a little simpler.
You need to isolate parts of the circuit until you find the fault. I would ,
1) discon everything at the breaker to test the breaker.
2 then discon the individual buildings one at a time at the box where you enter to decide if the fault is underground or in the building.
3) When you determine if in say building #2 there is a fault ,then you start at the end of building #2 and systematically discon stuff there until you locate the fault(either a pinched wire in a wall between box 6&7 or a bare wire in a box touching another or something will appear.