A country near the "ice wall", It is possible to fly from a location in that area to another country, on the other side of the "flat earth" without doing a 180.
You can just go straight the entirity of the trip without ever colliding with this wall or changing direction.
Ok let's look at a specific example. Let's say you want to fly from Brazil to Australia. Here are the ways you could do that on the round earth:
1) You could fly due east. You would travel around to the opposite side of the earth, and land in Australia.
2) You could fly due west. You would travel around to the opposite side of the earth, and land in Australia.
3) You could fly due north until you reach the north pole, at which point you would have to fly south (because you can't get any more "north" than the north pole). Once you got to the north pole, you'd fly south in the direction of Australia, and you'd get there eventually.
4) You could fly due south until you reached the south pole. Since you can't go any further south, you'd have to start flying north. You'd fly north in the direction of Australia, and eventually you'd get there.
Now here's the kicker:
Every one of those scenarios is, with the exception of number 4,
exactly the same for a flat earth. What about #4? Well, pilots don't do that, so I'm not worried about it. Then again they don't do #3 either, but that still works.
Here's what you're saying:
"Based on the assumption that the earth is a sphere, you could fly south from Brazil over antarctica and then north to Australia".
The problem is that this is based on the assumption that the Earth is spheroid, which obviously cannot be taken as given. The only things you can argue are, for example, what commercial planes do, or if I wanted to get picky, what you've done yourself. Commercial jets don't fly over antarctica. They might fly due east, due west, north then south, or anything else, and all of those scenarios fit perfectly with the idea of a flat earth.