Locking out advance with welds

The cap is fixed in relation to the distributor body, the rotor fixed in relation to the upper distributor shaft.
Nope, you have to rethink that. The rotor is fixed in relation to reluctor, Ignition happens when trailing edge of reluctor tooth is aligned with pickup nub. With the pickup fixed to body, the phase will be the same. It does not matter if you rotate the shaft, or body. But if you move the pickup, the phase changes, because the cap terminals are referenced to body.
Perhaps Greg can understand, and then use better words to help explain.
No, you are confusing ignition phasing with rotor phasing.