So after thinking about your observations on gravity and floating about some more, I believe that the floating is because the same mass is above you as is below. therefore as you get closer to the centre there is more mass above you and less below you it slows you down. I'm pretty sure this would be difficult to replicate because earth has different density layers, while those in the game are the same density all the way through.
As for your question about gravity getting less as you get higher, this confused me for a long time. From my understanding its because less mass is in your sphere of influence, so if you were to stand on the surface and make an imaginary sphere around the planet your on, the gravity is at its highest. If you were then to go 200 miles up and make the same sphere, 200 miles up from the surface, then there would be less mass as a ratio in your sphere of influence. So as you went higher and increased the area inside the sphere, the ratio of mass/density to no mass/density would decrease therefore less gravitational pull.
As for the asteroid question I have no clue lol.