This is a very interesting theory! The unusual shape of the continent definitely does spark some fantastical thoughts, hahah. The creation of the Red Line could definitely be artificial, I don't have any counterpoints to that theory and I actually quite like it. However, I do have a problem to propose for the idea of an artificial grand line (and/or calm belt). And that is the small island Little Garden. As you can read here, Little Garden is a prehistoric island, which means it has existed before humanity started recording history, and it was able to maintain prehistoric climate due to its position in the grand line. This means that the weird nature of the grand line must have been active since prehistoric times. At the very least local to Little Garden. I don't think humans existed back when Little Garden started being a dinosaur's habitat, but if they did, they probably didn't have access to or knowledge about devil fruits yet. Since this is canon information, I have my doubts about the grand line being made artificially. And if the grand line itself, the weirdest phenomenon in the One Piece world, wasn't made artificially, then a linear continent like the Red Line could very well be natural too. Those are my thoughts up until now!