I'm confused. Are we talking about "the possibility of a flat Earth" or are we talking about distinguishing frames?
The possibility of a FE is permitted by the equivalence of gravity with continual acceleration, so both.
Wrong. We're talking about the possibility of a flat Earth.
Have you considered the possibility that the bulk of the FE disc is rendered non-gravitating by interaction with the UA field/mechanism?
I don't know what any of that is. I suspect you made it up...
This would preserve gravitation between masses on the surface (such as torsion pendulums), while allowing the FE disc to retain its structure.
In which case you're making a special pleading that gravity can somehow be turned off. Once you turn gravity off, you turn the tides off.
Is this something Einstein showed?