AC power measurement query
why is this line grounded?


The line isn't grounded to proper earth - it's just an internal reference node that happens to be called ground - it could easily have been called 0V or some other obscure title. It doesn't mean it should be galvanically connected to AC mains earth. That would be plain wrong.


  Also since the shunt method is inherently unsafe....


The shunt method IS NOT inherently unsafe but you do need to take precautions and I wouldn't advise this project for anyone who doesn't understand the implications.


  why is there a 0R resistor on the Neutral line


The 0R resistor is probably there to act like a fuse (0603 SMD) should someone do something incorrect (like think they can solidly earth the electronics.).