The EOS is wired across the hold lugs on a series wound coil and needs to be clean and making good contact for the flipper to flip. EOS also needs to be open to check coil resistance (just slip a business card between the contacts).

Have you tried filing the contacts of your cab switch? Just checking continuity across high current switches like an EOS or cab switch doesn't necessarily mean it will work as intended in circuit.

If that doesn't help, I would recommend rebuilding J2 of the driver board. Grounding out the flipper return wire in the backbox indicates everything is fine upstream and your Siegecraft tester shows the relay is giving the returns a path to ground. This would make me suspect something might be flakey in the connector. And while you have the board out, reflow solder to the relay leads.

The flipper works when he grounds the return at J2 in the backbox which makes me think it's the driver board connector, not the one between the lower cab and backbox.

Paul, I'd recommend straight up replacing the header pins @ J2 with a modern .156" connector instead of just reflowing solder. The skinny round ones WMS used at this time were awful.

Nice work, man!