how does the mass of a body behave if it were concentrated?

Answer 1

If the line between the centers of mass of two bodies is taken as an axis, the orthogonal components of all other points in the bodies sum to zero.

Answer 2

Not sure what you mean.

If you mean how does an object behave if its mass compressed to a smaller volume, then the only change is in the surface gravity of the object.
The gravity of an object is directly related to the mass, and the gravity of a large object such as the moon or the Earth acts as though all the mass were at a single point in the geometric centre.
Compress the mass into a smaller volume, and the total gravity doesn't change.
But surface gravity is inversely proportional to the square of the radius - a smaller volume means a smaller radius, so the surface gravity will increase.