That "Ultimate Computer" engine room has to be in the primary hull because of the long curving hallway walk to the engine room and the same walk leaving the engine room."
No, it does not have
to be in the primary / saucer hull which is exactly the point these WIP deck plans try to illustrate and I addressed the circular corridor radius issue in the introduction.
1. According to the known Season Two studio set plan this would put the UC engine room in the center of the saucer's stern and pretty much block any kind of turbo lift travel between the saucer and the engineering hull (regardless whether you prefer a horizontal/vertical turbo shaft design or the diagonal one I'm advocating).
2. There would be a noticable height difference (which you discovered, BTW) between the Impulse Engine Room(s) from Season One (which do leave turbo shaft space and look like a foresighted Matt Jefferies intention) like "Court-Martial" and a mirrored Season Two engine room. Not only would the height differences look strange but the corridor allignment would be more severely compromised than in the reduced radius concept I'm proposing for the engineering hull around the engineering hull's antimatter pod (where also the circular engineering hull corridors seen in Season One retain their obvious and essential circular nature).
3. The UC engine room features the floor structure commonly associated with the dilithium crystal converter assembly and/or the matter-antimatter reactors. As you brilliantly concluded two of these reactors are in the nacelles and one is inside the engineering hull. It seems to make sense to have this kind of floor structure rather close to any of these three reactors than up in the comparably remote saucer hull.