Why do you only have a 2:1 welding transformer ? The spot welders I have seen have a single turn secondary !
I never said I have 2:1 welding transformer my transformer are 20:1 to 50:1 and usually have 1 turn in secondary In my earlier mail I have clearly mentioned The welding transformers have a turns ratio of 20 to 50.

Why not measure the resistance of the joint, with a pre-weld measurement, as the joint is clamped,then apply your current-to-suit, using phase control to adjust as needed

Nice Idea but the resistance of the joint is not the only parameter affecting current there are several others such as voltage fluctuations, temperature.

Further the resistance of job is not the plain resistance but a contact resistance. and this resistance depends on the pressure applied through electrodes and keeps changing as the temperature of junction changes.

The temperature is again a function of several parameters such as cooling water temperature its flow rate etc. Pressure of electrodes depends on the pressure of compressed air what if the air pressure changes during welding.

Regards,
Prahlad Purohit