I don't think the Drexler cutaway depicts that area as "uninhabitable." If you look closely, there are some gold outlines of door panels and curved struts that match the shape of outlines in the crew quarters, briefing rooms, etc. in the saucer. That implies there's some kind of occupied area there.
Also, in "Dagger of the Mind," there's a reference in dialogue to section C, deck 14, and both the Drexler cutaway and the FJ blueprints seem to agree that deck 14 would be the lowermost deck of the dorsal. (And the scene where a crewmember spots Van Gelder on that deck only shows a small section of the corridors, so it's not too inconsistent with that possibility.) "Mudd's Women" also implies that the captain's quarters are on deck 12, which would also be in the dorsal, but since that was later contradicted, it's best disregarded as a continuity glitch.