Gravity? What's become of the UA? But anyway, surely gravity would decrease on an inverse square law as you move away from the plane of the flat Earth?

Not if it is an infinite flat Earth.

One easy way to think of it is as a ball, but have the distance be a function of the radius.

So for example, surface gravity is 9.8 m/s^2, and that is at a distance of 1, the radius of Earth.

To have it decrease to 1/4 of that you need to go to a distance of 2.

For the RE that is ~6371 km above the surface, as the radius is ~6371 km.

For an infinite FE (approximated as a ball), that is ~∞ km above the surface, as the radius is ~∞ km.

Either way, we know the atmospheric pressure at various altitudes and it drops as expected.

For the altitudes we "know" (i.e. those which don't rely upon space which would just allow us to use photos of Earth) the value of g used is basically the same.

We sorted that in another thread. The sun has a lamp shade around it so the whole FE doesn't get lit.

If that was the case it would just vanish high in the sky as the shade obscures it.