You gave up and wired every trace. I know that feel. You really shouldn't have to, all the ground lines are tied together on the back of the motherboard.
You would think that would be the case. But after I took those pics I started disconnecting the ground wires one by one and testing after every disconnect. Low and IGNORE, second ground pin I disconnected the system would no longer boot (pin 6). Sane thing occured as I proceeded down the line of "ground" pins.
Now the only explanation I can think of here is that the pins we think are ground on the MB itself are in fact not ground, but very low resistance connections to something or other that go to the ram chips and must be connected to ground by the jumper pack to make the system boot properly.
I may be wrong, but its the only explanation I have for now based on the limited testing I've done.