I personally think that we should all refer to Robotsteve's argument. He seems the most levelheaded in the forum. Narc, you should listen to your fellow FEr in this case.
Asking which way is "up" on a Round Earth is like asking which way is "out" from the centre of a ball. If you attempt to use this reasoning to prove that a Round Earth can't exist, then one could also attempt to use it to show that a ball can't exist.
One has to remember words can have arbitrary meanings attached to them, especially in a different frame of reference. We simply assign common meanings to them
IF YOU THINK OF THINGS THE SAME WAY. If you say, "hey look, this is a GREEN apple" to a colour blind person, what you're seeing is not the same as what they're seeing. Similarly saying the same sentence makes even less sense if you say it to someone who only understands, say, French.
In FE, "up" is defined as the direction of Earth acceleration. In this case,
EXPRESSION = DEFINITIONIn RE, "up" is defined as the vector formed by starting at the observer and continuing with the sky. So yes, the
EXPRESSION of what "up" is changes depending on location but the
DEFINITION does not.
Just because one cannot determine the orientation all the time is meaningless. Using Narc's example only:
Nobody can tell me the orientation of an FE Earth model in respect to the galaxy / the entirety of Space (I believe even your FAQ mentions this possibility; that only our local known space is known to move your definition of "up"). There may be an another Flat Planet that is moving the opposite direction. Their definition of up, according to you, would contradict ours. You must admit the possibility of this alternate planet if you believe in Newtonian physics: Anything action (force) has an equal and opposite reaction ("negative" force). The energy to move a FE earth "up" (your def) must necessitate energy moving in the opposite direction.
Either way, any arguments using these definitions of "up" to prove either FE or RE is flawed, as numerous people have pointed out on both RE / FE sides. There's no relationship between the two definitions of "up", so you can't (easily) use one definition of "up" to prove itself or disprove the opposite.