Double the distance between them and the force with be less by a factor of 4. Newton found a physical basis for all of Kepler's laws. He used his laws of gravitation and motion and derived Kepler's laws of planetary motion. He also found that Kepler's first two laws apply to not just planets but any mass orbiting any other mass under the force of gravity - moons around planets, stars around other stars, etc. It is possible to have orbits that are not elliptical.
Comets generally follow parabolic orbits.
Newton generalized Kepler's 3 rd law so it would apply not just to our solar system but, again, to any masses in orbit. He showed that the generalization of Kepler's 3 rd law could be used to find the masses of the bodies in orbit and what they orbit around. With this law he could measure any orbiting body in the Universe. We use this law today to measure the masses of binary stars, of extrasolar planets, and of whole galaxies. They started by trying to explain local phenomena and wound up being applied to the Universe.