Sill Seal, corners during beef up

A couple questions. I'm planning for a garage conversion. Slab on grade, double stud wall, 2 layers of 5/8" sheet rock w/ GG. 1) When I build partition and inner walls, should they sit on anything other than sill seal? Is flanking from a slab foundation into walls a major concern? I understand that a slab foundation is a good dampener, and I'm not planning to float a floor. Just want to make sure that I don't need to do something to float the walls to maintain isolation. If I don't have to buy any more expensive isolation products, that would be ideal, as I already see I am going to go over budget (and I've barely started). 2) Looking at how I will do the beef up stage. There are corners in the garage where the studs (structural) leave small cavities that I won't be able to get into (without taking the studs apart more than I want to) to apply beef up sheet rock. These are small areas, perhaps 4" X 4". The rest of the building will be beefed up in 2X6 external stud wall with 2 layers 5/8" and GG. How should I deal with these corners. I've thought of just getting some spray foam, filling them as much as I can, then on the outside applying multiple sheet rock layers (and acoustical caulk) to cover the opening into the corners, thus attempting to make them air tight. Because of the way the foundation is there will be at least a 3" air gap between the outer and inner stud walls, so there would be plenty of room for multiple layers of sheet rock if necessary. This would break the 2 leaf rule, but only for a small area, and I figured the spray foam would at least fill the cavity, and the sheetrock would apply extra mass to these areas, and help seal it. Does this make sense?

See if you can stuff some insulation in the corners, loose type. That spray in foam isn't of much use for what you are doing. With 2X6 walls, I'd see if I could get something in there even if I had to work a little harder.