Sorry, I know more questions than answers. That board that presents the connector similar to the ESTOP connector - there's three wires - L, N and FG wired up. Is there ANYTHING else wired to that board?

This is really looking like a FrankenSyil machine. That board ("Main Board") is the primary supplies for the rest of the "normal" X4 as well as the 3-phase controller for the brushless spindle motor. None of that appears to be wired. If that's the case, it's not necessarily a bad thing but might help us understand how much of the standard Syil configuration can be ignored. Right now I'm half convinced you can lose that board.