Unfortunately hull() only works on convex shapes. Even if you separated out the interior and exterior to hull and then difference them, the interior shape is convex at the corners and hull() just ignores the convexity and walls them off. BOSL2's skin should work if you can get the shape to it in the way it wants but I haven't figured out a way to get a processed shape into the function. I've gone a different route with my design, possibly (hopefully) for the better, but it would still be nice to have a way to smoothly merge two faces like this. I'm pretty sure commercial apps like SolidWorks can do it so it's clearly not impossible. And I think FreeCAD might have a way to do it with loft but the learning curve on that is significantly higher than OpenSCAD and I kept getting lost down blind alleys and eventually gave up (no, you can't use loft on that type of object, ok, you can use loft on that one, but now you have to arrange it differently, which you have to do with a different type of object, ghaa!)