Assuming that it is the uppermost hard ceiling that constitutes the room ratio - then you could absolutely use that interstitial space into for low frequency absorption.....
To answer your questions from the figure in the book (no problem posting it here) in corresponding order:
1) This particular project was a garage conversion - and the garage was attached to the home - the uppermost double layer of drywall in question was the inside surface of an airlock entry to the studio (that also allowed entrance to the living space behind the garage and the house). The airlock also acted to provide a wee bit more isolation to the studio from the living area.
2. That line is the inside edge of the plates for that little wing wall that acts as the inside edge of the bass trap (and is also a continuation of the inside face of the room itself),
3. OK - the horizontal 2x4 members are physically attached (nailed or screwed) into the studs behind them......... The 1x wood finishes over them are a carefully designed room treatment that are not just architectural in nature (although they do look great when all is said and done....... they create a Helmholtz trap as well as acting to add diffusion to the space.
4. Those are 2 wall frames - separated by a 1" air space - the drywall is located outside of the wall surfaces - thus there are only 2 leafs.....
6. Yes that is a hard connection between those walls - the wall to the right hand side of the picture (running north/south if you will) is the outside wall in the front of the garage) the hard connection is not an issues because it does not connect anything to the inside structure - the 2 are totally isolated. The interstitial space you highlighted in yellow is (part of) the air space between the inner and outer walls. The line your arrow points to in the inside face of the outside garage wall - the line immediately to the left of that is the inside face of the foundation wall below - which forced me to shift the studio wall to the left of that inboard from where it might normally be.
7. Yes you are - again - starting from the right hand side of the detail - outside face of the exterior wall frame - inside face of that frame - inside face of the foundation wall below - 1" air space - outside face of the inner wall frame - inside face of the inner wall frame - layer of 1/2" osb, double layer of 5/8" gypsum drywall - 1 1/2" rigid fiberglass between 2x4 horizontal furring - interior wood slat wall finish.........
I hope this helped,