I started this thread, but even I can't read through the umpty-nine comments and makes sense of all the (many excellent) comments. One thought I have -- and it very well may have been brought up by others, and even by myself, but I can't read it all:
What if the secondary hull is 90 degrees off from the primary hull? That is, it's basically a tapering tube. It could be filled with curved hallways that -- to the viewer -- appear to be arranged vertically rather than horizontally, stacked from right to left, tapering toward the shuttle bay. Think of it as a deck of cards stacked on end, from right to left, rather than top to bottom.
All it would take is flipping the gravity. There is no "up" or "down" in space anyway -- it's all artificial. Turbolifts could easily flip 90 degrees before arriving in the secondary hull, and flip again when reaching the shuttle bay (which obviously shares the viewer's "up" and "down").
Presto, all curved hallways in the secondary hull would then be easily explained.
While there's no evidence of deck plans, we do have a cutaway drawing reproduced in The Making of Star Trek.
Rotating the decks in the secondary hull 90 degrees, as you and at least one other in this thread have suggested, would be contrary to that cutaway drawing.