The only issue I see with leaving a gap is the triple leaf effect. Between layers any air gap non damped with material is going to create a space for sound to resonate and actually lower the stc.

Line the osb with a soundmat then drywall. Bunker. Yet flex in drywall. This ties the floor assembly logic you mention into a wall. You should be able to find reliable data on the idea or similar

Very important to understand your home vapour flow before sealing a room with non permeable membranes. Ensure your home is dry and know moisture always dries to the outside in cold climates. Below grade, be sure you have conditioned air movement and low humidity. Any moisture between the mlv and osb will dry slowly. Ensure this space never gets cold/moist to reduce chances of condensation.

https://www.usg.com/content/usgcom/...nt-prep/sound-absorbing-mats-boards.html

Page 10 info. Wood wall assemblies page 20.
http://www.usg.com/content/dam/USG_...ssets/acoustical-assemblies-en-SA200.pdf