After reading all this, the problem statement sounds like;
When player 3 is connected, the other three displays show bad information.
Correct?
And you rotated all displays into slot 3 and the problem stays with slot 3?
And you are correct, most old Ballys did not use ribbon cables to the displays. That does not rule out an issue on the CPU, but you state 2 MPU's. What was the original problem with the glass displays that made you update? Did it have the same problem?