Maine's giant spinning ice disc looks like it's reforming

Vortices in a near-2d-flow like a river will absolutely be circular, vortices need to be coherent in some plane in order to sustain themselves.

The specific circumstances here must be that that the water is on the edge of freezing during downstream flow and there is a naturally-very-long-lived vortex in one region for it to initially form the ice-disc. Must also be followed by some very-gently-bounceing-off-the-edges, and the river never getting too thin for it to get stuck.

“a body initially at rest relative to the Earth at altitude 300km experiences an acceleration of 0.89g towards Earth”

which is false in the general-relativistic way of looking at things. An accelerometer initially at rest at 300km would show no acceleration as it starts to fall towards earth. If the atmosphere and the earth could be removed without affecting the shape of spacetime the accelerometer would continue speeding up till it reached where the center of the earth was, then slow down, reaching a speed of zero at a point opposite from where it started, at which point it would repeat the motion in reverse. At no point would it register an acceleration. Nor would an accelerometer in orbit register an acceleration. In the general-relativistic framework, free fall means no acceleration, and the motion of a body in free fall is determined solely by the shape of spacetime. So, at best you can say,

“The shape of spacetime at 300km affects a body's motion the same way an acceleration of 0.89g towards Earth would if spacetime were flat.”

By the way, an accelerometer on the surface of the earth, e.g., the one in your smartphone, registers a constant acceleration of 1.0g in the up direction caused by the normal force of whatever the smartphone is resting on. If someone knocks the phone off a table, the acceleration becomes zero till it hits the floor. (Air resistance can also apply an (upward) acceleration to the smartphone, but is negligible at the speeds reached during a fall of only a few feet by an object as heavy and dense as a smart phone.)

Correct, which is what I said (just not as explicit). I now realize that the “you” could be ambiguous though, the intended “you” is the person performing the experiment doesn’t need to be themselves in zero gravity.

Also agree with this as well (I again just wasn’t as detailed in my response). I used the word apparent because that’s the established convention from all scientific papers on the topic, but you are 100% correct that it’s identical. The VomitComet flights are a great example, but a lesser known example is going downhill on a roller coaster.