Dave,
Don't you just love all the 'free' advice you are getting (me included)?
-Joe
Means we all want to see a part 2, where Dave actually fixes that broken LeCroy.
I also suspect that hole as the culprit, as many people here also do. Maybe the 3.3V power supply layer on one of the inner layers has a very small clearance to the plating of that hole. And as you can see in the video the screw has somehow fractured the plating and then maybe pressed the splitters against this 3,3V power supply layer, shorting it out to ground. Because when you look at that screw hole closely, you can see, that the PCB is there somehow de- laminated.
That wouldn't surprise me, because if you watch Dave's first video, you can see that this lecroy has some really big damage to the side of the case. This indicates that it may have been dropped. So that would put very high stresses on the PCB, particularly at the points where it is screwed to the case.
I would recommend to drill out the plating of that hole first, and then see if something changes.
The high current experiment Dave performed also tells me, that if that screw hole has somewhat to do with that problem, then it can't be a dead short. Because then the 8 Chips wouldn't have gotten warm, which means that they were actually powered up.
Also a easy method check, if some sort of high current action is going on around that screw hole, is to test the temperature with a finger. If there are significant amounts of current flowing trough a resistive fault (because the screw hole can't be a complete short because of the above mentioned reason) then it has to get warm. A easy calculation confirms that: Let's assume the circuit consumes 6A in normal mode, and also assume that all the chips are OK. So if there are 11A going in, then trough the fault, 5A have to flow. Even at a resistance of 0.15Ohms at 5A this equals 3.75W of power- dissipation. This screw hole has to get warm, even if there are big power Planes on that PCB.