I really must get around to finishing my set of deck plans one of these days!What I concluded from this episode on ship layout:
YMVV
- The Klingons control Deck 6 and starboard Deck 7; they started in the Crew Lounge suggesting this was on Deck 6 or 7; Kirk escapes and takes a turbolift to Sickbay suggesting Sickbay in not on that deck. (Other episodes suggest Sickbay is on Deck 5.)
- The majority of the crew are trapped below decks, suggesting Decks 8 and below which completely includes the Engineering Hull. I assumed the Entity influenced the crew to go to these lower levels prior to sealing the bulkheads. If true, then Main Engineering is not in the Engineering Hull, rather it is in the Saucer. (Unless there are two Engineering Rooms that look the same, one in the Saucer and one in the Engineering Hull.)
- Mara leaves main Engineering to go to the main life-support couplings on Deck 6; this suggests that Main Engineering is not on Deck 6. Coupled with the above statement that the Klingons control Deck 6 and part of Deck 7, so, I conclude that Main Engineering (two story set) is on Deck 7 (with the upper level extending into Deck 6). To further confuse the issue, at the end of the episode, the Entity leaves Main Engineering and seems to directly exit the ship from the fore middle level of the Engineering Hull, so, did the Entity trip and fall down a turboshaft from the saucer into the Engineering Hull on the way out?
- Kirk used the Transporter Room to beam into Main Engineering. If we are to believe that the Klingons really control Deck 6 and starboard Deck 7, then the Transporter Room has to be on Deck 5 or above, or on the port side of Deck 7. Of course, there's the old argument for multiple Transporter Rooms... I prefer two Transporter Rooms both on Deck 7 (one on the port side and one on the starboard side, but that's just my opinion.
.

Just one thought:
The majority of the crew are trapped below decks, suggesting Decks 8 and below which completely includes the Engineering Hull. I assumed the Entity influenced the crew to go to these lower levels prior to sealing the bulkheads. If true, then Main Engineering is not in the Engineering Hull, rather it is in the Saucer. (Unless there are two Engineering Rooms that look the same, one in the Saucer and one in the Engineering Hull.)
When Uhura makes her statement she is on the Bridge, where technically all the other decks are the "lower levels". So this leaves open the option for the entity to have trapped the majority of the crew in the outer areas of the saucer (where I presume most of the crew quarters are). It would be easy for the entity to leave bulkheads open to enable the "Fighting 38s" to access key areas of the ship such as Engineering, even if it is in the secondary hull.