But NASA cares about delta v most. Although that would make Mars closer. So perhaps closest approach is thought to be what laymen mean by closest.
or maybe that the “low orbit” distance given for Venus is 400km vs 250km for Earth.
No, that won't make much of a difference: the gain is basically just the gain in potential energy for the extra 150 km, which is equivalent to only a few tens of meters per second delta-v.
The ground is pretty much impossible to reach anyway, and not as interesting as the atmosphere about 50 km up, where temperature and pressure are about the same as on Earth and where breathable air is buoyant.
Yes, but I don't think the numbers given in the chart consider such possibilities. I think they're for a standard rocket blasting off from the surface and having to account for atmospheric drag when determining how much delta-v is required to reach orbit.
Darn it, I have just finished my burns.