Exterior leaf on its way...ideas for interior leaf?

[Seeker of Rock]

I'll post a recent photo in a bit. Several of you have been helping me on this and it is much appreciated. I have an exterior CBS leaf, 4" detached (not coupled to the exterior leaf) concrete floor slab (rough finish) with an average of 6-12" of space between the slab edges and the CBS walls. I am framing and pouring another concrete header for possible future windows (when we sell the house someday, I want to be able to convert this into a workshop, guest room, etc., so I'm pouring the headers so the infrastructure for windows is in place) today. One more course of half block on top of that, then the wood ledger for the roof structure begins.
Undecideds at this point are:

1) slab is uncoupled and I plan on Rick's advice to leave it that way. I need to even and pour the concrete edges so I don't have a 12" gap in some places. What is the minimum separation distance I need from the slab edges to the exterior leaf walls?

2) should I use baseboard trim to conceal this distance or just leave an open crack on the interior of the room perimeter?

3) what is the least expensive, yet effective, way to build the interior walls so they are not passing sound to the exterior walls?

4) the interior ceiling...as Rick pointed out, this is the weak link as to what I have planned so far, which is a pitched ceiling with 2x8 rafters, insulation between them and I had planned on a wood or drywall finish. Exterior roof structure will be 5/8" plywood support by 2x8 rafters, tarpaper, roll shingles horizontally, and mortared-in-place concrete barrel tile.

The goal is to maximize transmission loss to the surrounding neighbors, at the same time create a workable recording/mixing room. Interior dimensions at the interior block are12'4"x13'9", approximately 7'9" to the bottom of the future rafters, pitching up to about 10+' in the center.

Any additional help is appreciated.

 

[Seeker of Rock]

Hmmm.....#1 and #2, thinking about a bituminous pre-formed expansion joint, which I need anyway at a vertical/horizontal concrete union, then reducing the gap of the slab to 3/4" for the expansion joint. I doubt it will pass vibrations very well, and slab/wall will still be decoupled as the purpose of placing expansion joints between vertical and horizontal planes is to decouple them.
#3-I wonder if building a series of clouds that hang below the drywall ceiling would be the solution. The concrete roof tile on mortar will likely provide a lot of transmission loss from the interior, don't see a problem there. The problem seems that all of that sound is now contained in the room, so maybe the hanging clouds are the solution here.