Um, that's not quite how orbital mechanics works. Helios II started off from the earth - that is, with a lot of potential energy in the sun's gravitational field. It was put into an elliptical orbit around the sun, so on its closest approach, part of that potential energy was converted to kinetic energy (hence the high velocity), but at the most distant point, it's all converted back to potential energy, and there's zero gain. It's a bit like bouncing a rubber ball off the floor: it's surely going to hit the ground at a high velocity, but it's never going to bounce up higher than it started - no free lunch. The reason why slingshotting between planets works is because they move relative to each other. To use the rubber ball analogy again, if you throw a rubber ball at the front of a truck that is rapidly approaching you, the rubber ball will come back with a higher velocity. What you did is subtracting kinetic energy from the truck, just like a slingshotting probe subtracts kinetic energy from a moving planet. The tricks we use to make space probes gain kinetic energy are not unlike bouncing a rubber ball repeatedly between moving walls. To use the sun for slingshotting, one would require a very massive object in a highly eccentric orbit around the sun as a "second wall", which our solar system unfortunately doesn't have. (Or should that be: fortunately for our existence?) One could try to use pluto, but I doubt it's massive, eccentric and fast enough to be worth it.

Disclaimer: the above explanation is obviously somewhat oversimplified.

No, you do mean the inverse square law. The inverse square law states that the intensity of radio waves (or light) decreases with the square of the distance to the source. The square-cube law is about the surface relative to the volume: The surface increases with the square of the size, while the volume increases with the cube. This means that the same structure at different sizes would not have the same properties. For example, a giant ant would not be able to get enough oxygen, as its ability to get oxygen follows its surface area, while its oxygen demand follows its volume. Likewise, it would be crushed under the weight of its own exoskeleton, as its strength is proportional to the square of the size, while its weight is proportional to the cube.