...Also, Scotty's engineering responsibilities do seem to take him around. The existence of multiple engine rooms has already been discussed: if we're to believe that there can be no curved corridors in the secondary hull, then all the episodes showing the connection between the engineering set and the curved corridor set necessarily mean that Scotty spends time in the saucer.
On the other hand, we already have to rather fundamentally accept that the curvature of the set we see cannot match the curvature of all the corridors it supposedly portrays. If the set only has one curvature, it can only have one radius, making logistics in the saucer rather unbearable.
On the other hand, curving corridors need not mean circular ones, as later seen in VOY. The ship's outer hull is curved at all locations, including the secondary hull and the connecting neck. So even if we go by Peter's adage of sticking 100% to what we see, and not deviating even ten degrees, we might insert the curved set in places we find advantageous
without centering the curve on the saucer axis... Too bad that the engineering set is at such an acute angle wrt the corridor!
One thing to note is that aux. control doesn't appear to be laid out on the same ergonomic principles as the main bridge (regardless of whether we find those principles sensible or not). We get the general impression of a semicircle, its flat back wall against a corridor; it would be rather reasonable to assume the facility faces forward, then, and sits somewhere forward in the hull (whichever hull that happens to be), rather than, say, at the center axis of the saucer. "Doomsday Machine" also has our heroes almost walking past the room where Decker sits, suggesting auxiliary control might feature a string of rooms along the corridor; the one with Decker and the big control pulpit might hold no special status among these, then.
So, how about a side-by-side trio of rooms that all have this console, and are basically "mission rooms" that can act as headquarters for a planetside survey or other complicated operation, giving the team leader direct access to key ship systems as needed by that specific mission? This would be their main raison d'etrê, and the characteristics dictated by this role would conveniently allow for an (emergency-only) secondary role as a fallback control center. We'd again be dealing with a maze of sorts, the "center" in the name essentially meaning "cluster" or "concentration" of auxiliary control facilities (and conveniently explaining minor set discrepancies).
Since the rooms would not exactly be gathering dust during non-emergencies, it would be all the more plausible for villains to wander in alongside the legitimate users and then effect a takeover.
Timo Saloniemi