And with Z1 set at low current and a constant load on the welder the scope should show one of the three phases higher than the others and not adjustable.
If R equals the DC coil's resistance and X equals the coil's impedance and XL equals the coil's inductive reactance, then X² = R² + (XL)². If the coil is primarily inductive reactance, an ohmmeter won't tell you much.
If you have the schematics for the other welders and they are still working, I'd start measuring whatever components in each one that doesn't show a schematic value for resistance or inductance. For this one I see 10 unmarked coils.
BTW, the resistance of VR1 will vary with the voltage applied to it.
BTW, MIG welders are described as Constant Voltage sources, so arc welders might be more like Constant Current sources, with the SR3 diode bridge acting like a current summing network and VR1 acting to limit open circuit voltage, which is theoretically infinite for a CC source. That's probably why the SR3 diodes have such a high PIV.