Once you enter the great gulf nothing can stop your fall into the core. The pit is everything below the mantle. If the sides of the pit is the lower mantle, then the pit includes the great gulf.
Heads are round but a beard can have corners. If you think this passage means that the Earth is flat and square, you also have to make the case that human heads are square. Good luck with that.
If my feet are at the end of my legs, then the coast is at the end of the land. The ends of the earth is simply where the land stops: it ceases, fails, has and end. The beach.
Depending on what English translation you're reading you may see this rendered as "the coasts of the earth," "the sides of the earth" or "the ends of the earth."
If the Earth has ends does that mean it has to be flat? No, because "end" is a translation of a word which means "extremity or furthest reaches of" in which case we're talking about the beach.
The time between Noah's flood and Peleg's tectonics was when the dispersal from Babel took place. From Babel small family groups spread out, figuratively speaking, to the ends of the Earth.
Brachiopods aren't an evolutionary dead-end which got wiped out in a mass extinction. Well, there was a mass extinction, but it didn't wipe out the Brachiopods. Many were buried in Noah's flood but many alive today.
The first air-breathing animals to appear in the stratigraphic column were the ones who just happened to be living on the beach or at the coast during Noah's flood.
The first vascular plants that were buried in Noah's flood are on top of the beach sand which is full of dead scorpions because they were growing in the dunes inland of the beach.
If you want to believe that earth is flat no one's going to stop you. However nothing in the Bible, either in the meaning of words or in the context of passages, requires you to believe that earth is flat and has an edge.