Mudd’s Women: Kirk’s cabin is on this deck (apparently) and has a decent stretch of corridor adjoining it.
The full corridor is seen only before Spock and the traveling circus enter the turbolift; after the turbolift ride (and thus supposedly after they reach Deck 12), we only see Kirk's cabin (the standard set), with the mood set by a brief glimpse of some utility corner or another behind a grillework.
Dagger Of The Mind: Here, Van Gelder is spotted on Deck 14, having (apparently) exited the Transporter Room and started roaming the corridors. There is not indication that he ever used a turbolift – if he had, wouldn’t he have gone somewhere more secluded? Or if his ultimate destination was in fact the bridge – why not there? To detour via the pylon neck is an odd choice, even for him!
Note, however, that our heroes were beaming cargo
. No doubt van Gelder was beamed up into the cargo transport center in the secondary hull, and was fighting his way up to the bridge where he despite his compromised state of mind knew he would meet the person of authority he needed to defeat Adams.
Enemy Within: Rand’s quarters are blatantly on this deck (G. T. Fisher calls for aid from an intercom) and even more corridor is visible prior to this on Evilkirk’s drunken ramble to her door. Regarding this ramble, why is Evilkirk even here? The presence of Rand’s cabin seems to take him almost by surprise, just a fortunate opportunity to be taken advantage of. I would postulate that he is going back to his cabin and just happens to pass her own along the way. In fact, at this point in the series, “Deck 12” is almost being treated as the main deck of the ship, hardly suitable if that deck were situated in the pylon neck.
Many oddities there, indeed. Note how Fisher doesn't complete his sentence - perhaps he's calling Deck 12, rather than calling from
Deck 12? After all, Rand yelled for him to specifically call Mr Spock, as if her primary concern here was to keep the incident from becoming public, and personal security came as a distant eleventh.
We can defend in multiple ways the idea that all the action is taking place on Deck 3, Section C after all. We have the door signage:
We have McCoy's door similarly indicated to be at "3F". We have reason to think the officers (and possibly their personal assistants) live in a fairly isolated part of the ship, rather than "main deck". We later see Spock living on Deck 3 in the upgraded ship. Deck 3 ought to feature enough floor space and curved structure to account for the witnessed corridors - and superstructures like that are classic locations for officers' accommodation in real ships... And both EvilKirk and Fisher might have business at and near Kirk's personal cabin, thus a reason to walk past Rand's.