Your problem is with I5, the 63 wire, brown yellow, originating at the MPU J2-13. Are you sure you carefully examined this wire thoroughly from the MPU connector onward? It's either a board problem or more likely there is a break somewhere. I would start by turning the MPU over and reflowing the J2 connector. Then I would run a temporary bipass wire from pin 13 to one of the switches and see if that return row starts working. If it does pull the wire out of the harness and find the break.

It's also possible the 6821 chip is bad, As a last resort swap U10 and U11. Hows the condition of the board? Any battery acid damage?