Wow…. To be honest, I never even considered this… but I have to say, it’s a very convincing argument. I grew up with the FJD blueprints and just sort of assumed he had inside knowledge. My tiny little mind is blown!To judge from that ceiling design, Matt Jefferies thought Main Engineering was in the secondary hull. And fairly far aft, where the hull is narrower, because the Engine Room ceiling seems to span the whole width.
And maybe he thought the caged pipes went up into the angled pylons that support the warp engines. That's been a fan theory at times, and in Jefferies' mind it might have been canon.