Essentially, I thought that's what I was doing but now I'm confused.
I have revised post 5, and what I did was
1. Created two independent paths from (x,y) to (x_0,y_0)
2. Let the two paths form a closed contour C, in which C=C_2-C_1
3. Since the integral of a closed contour is zero, then C_1=C_2...