This is actually my first post, because I am a V-Ray guy...
But we had exactly the same problems with dome light set to ground projection. Don't ask me how, but Vlado figured out a way to fix it. I don't have the original images anymore, but it looked exactly the same: Objects seemed to be partly transparent when they were near the ground.
As to the other three images: The first one looks correct to me. This is the behavior I would expect because the lower parts on the front bumper are curved inwards, it of course reflects the ground it is standing on.
The other ones look plainly wrong because the reflections of the environment are way too big. And why there is nothing reflecting in the lower parts of the bumper, is a mystery to me.
I envy you Corona guys for your HDRi and dome mode stuff, that is exactly what I would need in V-Ray. Because there, changing the radius only affects the ground projection, but not the rest of the environment above the horizon line...