Flat-earthers already "changed" the physics of light propagation. I guess they would have no troubles bulls*tting their way out of gravity as well because none of them has actually the math/physics/etc basics to prove themselves wrong (what's a lab anyway, a brainwashing tool of the lying elite?).
Can it actually be demonstrated in a lab that gravity non-planar? One interesting explanation I've seen by the flat earthers is that gravity does not exist, instead the flat earth is simply under constant acceleration at 9.8m/s^2.
Yes. Very precise measurements of gravity on Earth have shown that, although it always points to the Earth's center, the force of Earth's gravity varies by latitude a tiny bit[1] (because Earth is a rotating oblate spheroid) and by elevation (because gravity varies by distance between center of masses). Neither of these can be accounted for on an accelerating disk Earth.
[1] https://www.britannica.com/science/gravity-physics/Accelerat...
A flat shape is impossible because the gravitational field would be vastly different on a planar body than on a spherical one, as I go further from the center of mass the further I go towards the edge.
And how gravity works can easily be demonstrated in a lab setting.