The edge of the FE is one giant ring plate. As the outgoing plates contact this ring, they subduct at this location. As the plate subducts, it is heated, melted and recycled. Due to this subduction, the ice wall formed.
Wouldn't work. There would be a "surplus" of subduction, as on the FE map,
When the outgoing plates subduct into the antarctic plate, they end up back into the inner layer of the flat Earth.
the Antarctic plate would have a longer boundary than in the real world.
Yes, because it circles around the edge of the flat Earth.
There would have to be more divergent boundaries than actually exist.
The divergent boundaries move plates apart from each other, causing the plates to subduct into the Antarctic plate. Thus, the divergent boundaries (near the ice wall) circle around the flat Earth oceans. The plates subducted into the Antarctic plate ends up back into the Asthenospheric (upper mantle) layer of the flat Earth, which are recycled through melting.
Looks like I'm gonna have to make another diagram for this...