I disagree about the need for extensive structure, Cary
. This thing can be a semi-monocoque structure, with the main framework lying deep within the saucer and secondary hull. That framework might have little to do with reinforcing the shape against momentum shifts under normal operating conditions. Reinforced hull structures made of exotic materials might do that work, while deep framing might only be there to lend support to the skin under extreme, abnormal stress.
I'd include some minor structure towards that end in the neck, as well as some redundant connections between secondary hull's main fuel sources and primary hull reactors. I'd also suggest a second turboshaft to clear that tight traffic spot between the two hulls. Otherwise, I agree completely with using the remaining neck for FJ
-like public spaces. I'd also include VIP quarters (for special environments needs) and even temporary berthing for crew working in the upper secondary hull.