Beautiful and graphic visualizations.
Looks indeed like a very close call. Since I already lowered Deck 5 onto Deck 6 in the cutaway draft I could / should either lower it a little more and/or assume an average deck height between 2.7 - 2.8 meters.
Technically speaking there are more people on this planet that experienced TOS on a 4:3 TV tube set with trimming overscan than those that experienced the Blu-ray discs on a 1:1 pixel mapping flat screen - which is neither what the directors intended or foresaw (great, first TNG and now I'm having this kind of discussion for TOS).
As for the briefing room set, the walls extend beyond the upper beam which apparently was a guidance for the director to know how to compose for his safe area. "In-Universe" this empty space above the support
beams serves no meaningful purpose.
So when we do have a screencap that reveals empty space above those beams, what would the Thermians do? Deny the structural support performance of these beams and downgrade these to fancy decoration?
As for the "Level 2" subject, I remarked at another BBS thread longer ago, that the outer rim of Deck 7 is so detached from the actual inner Deck 7 that it rather appears to be a lower level of the outer rim of Deck 6, hence "Level 2" in ship's personnel lingo, IMHO.