Other thing is I believe they say gravity is strongest at the center of the sphere/core but I would think the mass is split evenly away from the core eg. maybe 2/3 radius from the center where it's equal mass on each side. But probably doesn't make sense wouldn't be a ball.

Since the strength is represented by the slope of the sheet (not the depth), it should still line up. Underneath the ball at the very center the sheet will be level, to match as you say, that the field strength is 0 there. The exact shape will probably be wrong though since it's mostly determined by the shape of the bottom of the object.