I think the fjord-like structures work when the backstory is taken into consideration. They'll erode away quite rapidly now that they're subject to tides, but in the short-term they make sense to me.
Speaking of that aquifer, I wonder how it will be affected by a sudden inrush of seawater? Obviously the freshwater will lay over the top of the salt, so after the seas begin to rise, it's entirely possible that the wells will start to overflow. Areas that were once semi-arid might suddenly become fruitful when the water table is temporarily supercharged by the saltwater being injected to the lowest levels. A bit like an oil well—pump water into the oil sands, and the lighter petroleum will be pushed upward.
I have no idea what the long-term effects of saltwater in an aquifer would be. For that matter, this entire post is purely speculative. I have no particular expertise in geology. But maybe it gives a place to start asking questions of people who do know something about it.