May be I did not got it right, but why make it so complicated?

You need current flow, so from the 0 point input to your regulator put the shunt resistor with an op-amp and then the ADC. Even better, Analog has such current sensors already made with op-amps and the rest in one chip (AD8210).

You don't have to mess with the regulation loop of the regulator and sense lines. Work on the input directly after your rectifier/capacitor.

Ioannis