Outside voltage is definitely making it into the switch matrix. You absolutely need to try and find out exactly where that is occurring before you waste your time swapping many more parts on the board as it will just keep happing as long as the problem still exist. I know you suspect the motor, and that is a possibility, but it is very difficult for the motor voltage to make its way into the switch matrix the way the circuit is designed. As opposed to coil voltage via a pop bumper, sling, etc, basically anywhere there is coil voltage and switch matrix voltage extremely close to eachother. Sometimes it is very obvious, but most of the time it is a tedious, frustrating search for the point in which two wires are touching where they are not supposed to be. As for your current switch issues, the only thing that can cause incorrect switches to actuate when a switch is activated, besides being wired incorrectly, is a logic issue. I would suspect that at one point in time the high voltage made it's way past the transistor in the switch drive and into the LS244 located at 8J. If not the LS244, then it may have gotten to the 6821 PIA located at 8H via the switch return. A logic probe would be a big help in determining whether the is the case for certain.