Still ignoring how magnetic fields actually work, Tom? Yes, they always form closed loops. But often they start off moving away from either pole before the curve back around, go past the other pole, and curve back once again to meet it. So yes, often enough magnetic fields would just fly off into nothing, with the effect being most pronounced in the vicinity of either pole. I can get the diagram out again if you like.
The problem you pointed out in the previous thread was that if you are directly below the South Pole and attempt to travel Northward, the 3D field lines might curve upwards on themselves overhead of the vessel, leaving the compass confused.
How many ships below the South Pole in Fiji are set their path to travel exactly North, though? Not many, I'd wager. Most will be traveling West, North West, East, or North East towards known landmasses, not out into open ocean.
But even if a vessel happened to venture out too far North from Fiji, it stands to reason that they would see their compass non-functional and decide to sail back towards the sun, either going back home, or perhaps sailing around until catching one of the stray NW or NE field lines which curves towards the North Pole. I doubt anyone would sail blindly into open waters without a working compass.
In all respects, the bi-polar model allows for navigation from almost all points on the map, except as you pointed out, when one wants to travel exactly North from below the South Pole, or when one wants to travel exactly South from above the North Pole. Unfortunately, it is an assumption that Fiji is directly South of the South Pole. As we do not have a confirmed map, for all we know there is just open ocean at that location. Perhaps Fiji is not directly centered beneath the South Pole as illustrated in the map but a little to the left or right. This would cause anyone traveling exactly Northward from that location to catch one of the field lines and curve around the world to the North Pole.
Hence, even if we did find a report of someone traveling exactly North from Fiji (unlikely), we do not even know where Fiji is in the model to say with confidence that the ship traveled exactly North from dead center beneath the South Pole.