Double drywall will increase the mass. Green Glue will add damping (conversion of acoustic energy to thermal energy as a means of increasing transmission loss). Are you planning to double this on the ceiling? What's happening at the sub-floor?
If you require the contractor to stagger the drywall seams you'll see some further minor reduction of airborne conduction as compared to doubling the drywall with the seams aligned. If you have them apply Green Glue sealant, which is a different product than Green Glue damping compound, to all edges (both horizontal and vertical) prior to mudding and taping you'll get more airborne reduction still. By omitting RC, or better yet hat channel mounted to Whisper Clips, you're sacrificing any sound isolation via decoupling. Your rendering also includes what looks like a fair amount of glass, and an HRV as you mentioned.
Doubling up will certainly help, particularly if you do what I described above and follow these practices through to the ceiling and the sub-floor (my sub-floor is drywall, then Green Glue, then topped with plywood, for instance), you'll improve your sound isolation a bit. Ultimately, I think the HRV and the windows are going to detract significantly from whatever you can do and doubling up the drywall won't be worth the effort. I mentioned this earlier in the thread, but your isolation needs are super minor. I would just build as a normal structure, finish it out in a pleasing way, and be mindful of your volume after hours.
What does this mean? Are you building an unvented hot roof? If so, have you confirmed that your local building codes and inspectors will allow that? I wanted to go that route, but Seattle's building department denied my application to do so and as a result I had to build a multi-leaf ceiling structure that added a bunch of cost, time, and complexity to the build. If you're not building a hot roof and it's vented either at the eaves or at the ridge, how are you addressing them for the isolation perimeter? If you haven't closed them off from the area where sound will be generated (i.e. just like typical construction where sound will enter an attic space after passing through drywall), then you'll have another major source of sound transmission, which would further reinforce my suggestion to not bother doubling up the drywall. Remember that a single leaf system is bad, a two leaf system is ideal, and a three leaf system is better than a single leaf but not as optimal as a two leaf.
If you do go forward with the double drywall, what are you going to do to isolate the HRV? At a minimum I would consider constructing a baffle box. This will reduce the efficiency of your HRV for the sake of sound isolation, so you'd likely need to upsize it to maintain the same number of air volume changes per hour that you've put into your design. Speaking of which, how many complete air volume transfers did you design when picking an HRV unit? If you're not going to sound isolate the HRV, then again... don't bother doing any of it.
